The entry for niti in Lewis and Short includes meaning I.B.3: "to strain for a stool". One passage of such use is mentioned:
Suet. Ves. 20:
Statura fuit quadrata, compactis firmisque membris, vultu veluti nitentis…
English translation (Alexander Thomson, ed., 1889):
He was broad-set, strong-limbed, and his features gave the idea of a man in the act of straining himself…
To me it seems as if this could be included in the meaning I.B.2, "to strain in giving birth". In general, niti seems to mean "to rest against something" or "to make an effort", and even that general idea might be enough here without specific reference to birth. The man's face was as if (veluti) he was giving birth. I would argue that niti used in this sense here, not referring to what the man was actually doing.
This leads me to a couple of questions:
What does "straining oneself" or "straining for a stool" actually mean? I get the impression that they are euphemisms for defecation in an attempt to avoid profanities, but I'm not sure if something else was intended. I had never heard of "straining for a stool" before. (More literal translations, however profane or offending, are often useful in getting the meaning and tone right. Bear in mind that English is not a native language for me.)
I argued above that the one in Suetonius doesn't even have that particular meaning — assuming I understood the English correctly. Do you agree with this judgement? How would you interpret the word in that passage?
Are there other examples of niti in meaning I.B.3? (Or are there any examples at all if you agreed with me?)
This question was inspired by brianpck's comment under Hugh's answer to this question.