What it means
Sheesh is the drawn-out exclamation you let out when something is impressive, shocking or a bit much, half awe and half playful disbelief. The longer you stretch the vowel, the bigger the reaction you are giving.
Usage examples
"Sheesh, you ran the whole marathon without stopping?"
"He pulled up in a brand new car, sheeesh."
"Sheeesh, that fit is clean, I can't even hate."
"You got front row tickets for twenty bucks? Sheesh, that's a madness."
Where it comes from
It’s an old English exclamation that’s been around in speech for ages as a softened way of reacting to surprise or exasperation, basically a cousin of geez and Jesus-type swears. What blew it up for younger speakers was the stretched-out sheeeesh meme on TikTok in the early 2020s.