Egregoros

@koakuma you are holding fast, being steadfast in your obligation, firmly devoted, trying hard not to eat that juicy meat. huge sausage tempting you, but you remain hard!

@koakuma Not from fast (quick) but from fasten, to hold fast (in abstainance).
Fun fact, coincidentally, French also has a weird homophone for fasting, jeune (young) and jeûne (fast).