Crabs For My Sensei is a meme that originated from the 5th OVA of One Punch Man airing in English Subtitles in April 2016.

During the train ride to City Z, Genos and Fubuki sit in adjacent seats, the seat across from Fubuki being boxes of live crabs for Genos to take home for dinner with Saitama. It is known that Genos acts a lot like a housewife to Saitama, so going out and getting groceries is a big deal for him.

After silence, Fubuki looks at the boxes of crabs rumbling in the seat. This is when, without hesitation or a change in his serious expression, Genos tells her-
"They're crabs. Crabs for my sensei."
Fubuki fails to respond to Genos' seriousness about how urgent these crabs are for his sensei.
The meme originated from Tumblr, meme posts getting as high as 5,000+ notes.