What does “suck it up and go” mean?

The idiom suck it up meaning to stoically endure hardship in order to achieve some objective, or meet expectations, seems to have started to gain currency in the mid-1970s.

A somewhat graphic etymology put forward in Urban dictionary is that it's pilot slang. If you vomit into your mask, you'd better suck it up. Otherwise, you can inhale it and die. I can't deny that may be true - but I'd rather not think about it too much!

Here's a Wiktionary "talk" page dismissing the WW2 pilot slang origin. Their actual entry for the phrase says it probably derives from "suck up one's chest", but I must say "suck in one's stomach" is more common for stand tall and straight, stoically ready to be judged/subjected to adversity. Whatever - I'll just say the origin is "uncertain".

The italicising of suck it up and go is by OP. The and go [to the baptism] isn't part of the idiom - it's just the hardship to be endured in this particular case (the aunt obviously doesn't want to go if she's still not to honoured as a godparent).

Obviously this idiom is unrelated to suck it and see (try out an idea). Interestingly, there's also suck it all in (to wholeheartedly embrace all aspects of a novel experience or environment). But to suck in the sense of "be inferior" also got started in the 70's, and is now ubiquitous, along with suck my dick. So quite likely "positive" idioms like suck it all in will fall into disuse because they clash with the more common negative usages.

The phrase suck it up means to put up with something, deal with with it, without complaining. Carolyn answers that she should deal with the fact that she isn't a godparent, not complain and silently attend the baptism as invited.

suck it up is akin to get over it or brace yourself. It implies resistance against some action but, if really desired, one could deal with it or just do it.

"Suck it up and go" means the same thing, but in this example it specifically refers to going to the baptism. The aunt feels snubbed and, therefore, sees the baptism very negatively. To "suck it up and go" means to put those feelings aside and approach the baptism as a positive thing in order to support her sister-in-law.

I always inferred that this was a metaphor for the act of hardening your abdominal muscles so that you could take a punch to the gut without flinching. (Not something I have any literal experience of! So I don't know for sure if sucking in your breath would do the trick, but that was my assumption.) The expression means to demonstrate toughness by enduring something unfair or unpleasant staunchly, with minimum fuss - because the act of doing so both preserves dignity and makes whatever it is easier to endure.

Lately I heard the aviation theory about what you would need to do if you vomited into your mask - I guess that works, but ... yeuchhhh!