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When someone is bad at throwing a ball in sports.
"Nice throw, custard arm."

"What is Jono doing, he's got the biggest custard arm ever."
by Jono P. November 24, 2005
Having fat arms, the fat usually hangs from the arm - the same as lunch lady arms
Man, check out that fat piece of shits custard arms
by pezzzzzz October 21, 2009