Genesis 38:5

And she added, and bare a son, and called his name Shela, because her husband had forgotten her and was in cessation when she bare him.

AND SHE CALLED HIS NAME SHELAH, AND HE [Judah] WAS AT CHEZIB, WHEN SHE BORE HIM. Rashi wrote: “I am of the opinion that because it was there that she ceased bearing children, the place was called Chezib (deceit). It is similar in expression to the verse, Wilt thou indeed be unto me as a deceitful (‘achzav’) brook. If this be not so, what is the verse teaching us by mentioning that Judah was in Chezib?”
Now I do not know why a place should be named for that reason, [i.e., because there she ceased bearing children], there being nothing outstanding in such an event as three sons were sufficient for her. Moreover, at the time she gave birth to the third son it was not yet known whether she had ceased bearing or would give birth afterwards. Only at the time of her demise did it become established [that she had ceased bearing with the third son].
Now some scholars say that it was their custom for the father to name the firstborn, and the mother the second one. It is for this reason that Scripture states concerning the first son, And he called his name, and concerning the second one, And she called. Now concerning the third son, [the naming of whom was the father’s prerogative, Scripture nevertheless] says, And she called, explaining that this was because Judah was in Chezib when she gave birth to him, and he was not there to name him. This interpretation lacks rhyme or reason.
In the opinion of Rabbi Abraham ibn Ezra Scripture relates where they were born; the statement, when she bore ‘him,’ is as if it had said ‘them,’ as all three sons were born in one place.
In my opinion, the name Shelah is an expression meaning a thing which stops and deceives. Thus, do not ‘thashleh’ (deceive) me, which the Targum there renders as, “Let not your word deceive your handmaid.” Perhaps it is related to the concept of error, for he who commits an error deceives his thinking. Thus Scripture is saying that she called him Shelah, [a word which is traceable to the root of the Hebrew word meaning ‘error,’] because of the name of the place, as he was in Chezib — [a word which means ‘deceive’] — when she bore him. And [the word v’hayah (and he was), although it should really be saying, v’haytha, (and she was), is identical with the expression ‘V’hayah hana’arah’ (And the damsel shall be). in the feminine, except that the word v’haya does not refer to na’arah but to the event itself and is therefore to be understood as: “And it shall come to pass that the damsel, etc.” Here, likewise, it is to be so understood. This is the intent of the saying of the Rabbis in Bereshith Rabbah: “Paskath was the name of the place.”

והיה בכזיב, Yehudah was at that location and that is where the child was born.

והיה בכזיב, the reason why she called their third son Shelah, a word reminiscent of Kings II 4,28 (לא) תשלה, “(do not) deceive, disappoint, disillusion,” a name which is not exactly complimentary to the bearer of it, was due to Yehudah being in כזיב, i.e. a reference to his being absent, so that she did not have this moral support when she began the labour pains prior to giving birth to her third son. [I am not sure if the author understood the Torah as using to the word כזיב to describe Yehudah’s state of mind, and that there was no such place at all. Ed.] Had he been there, he would never have agreed to the name she gave his son.

ותקרא את שמו שלה והיה בכזיב בלדתה אותו, “she called his name Shelah, as he (her husband) was in a place called Keziv at the time when she gave birth to him.” (standard explanation) The expression “shelah” and “keziv,” are identical in meaning. They appear in connection with inability to give birth, such as in Kings II 4,16 when the woman from Shunem being promised by the prophet Elisha that she would give birth to a son tells him not raise false hopes as she knew she was past the age when this was possible. The words she used were אל תכזב. When she upbraided the prophet after the son she did bear had died before growing up, she reminded him of what she had said then and used the verb שלה. (verse 28 in the same chapter) The word כזב meaning interruption in the sense of termination, also occurs in Isaiah 58,11: וכמוצא מים אשר לא יכזבו, “like a spring whose waters do not fail.” According to this, the name shelah indicated that his mother knew she would not bear any more children. [This leaves the question of why the masculine והיה, “he was,” is used in our verse Ed.] My teacher told me that כזיב is the place of a town or village. It was customary in those days that mother and father named their children alternately. The father would name the firstborn, so that it would have been Yehudah’s turn to name the son born third. Seeing that he was not at home when this son was born, the mother decided to name him instead. This is why the Torah explains why Yehudah had not named Shelah.