Mr. Selfridge, Episode 7

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’sstill 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 suckin the sense of “be inferior” also got started in the 70′s,