I was reading this New Statesman article and was surprised to read this:
The origin of the phrase “suck it up” is quite gross. Allegedly, it’s what WWII pilots were instructed to do if they vomited into their oxygen masks, to avoid drowning in their own puke.
I can find little evidence to support this. There is some talk on wiktionary and a related question on the English Language and Usage Stack Exchange.
This usage data seems to contradict it as there is very little change in usage over the WWII era. But I can't find a different origin.
The usage data could also be confused as there is a literal interpretation of the phrase. Looking at the google book results all earlier references seem to refer to a literal 'sucking up' where as post WWII they seem to include it as a metaphor.