1
general::
Pay, hand over, as in We had to shell out $1,000 for auto repairs. This expression transfers taking a seed such as a pea or nut out of its pod or shell to taking money out of one's pocket. [Colloquial; c. 1800]
American Heritage Idioms
2
general::
Phrase(s): shell out (an amount of money)
to spend a certain amount of money. • I’m not going to shell out $400 for that! • Come on. You owe me. Shell out!
McGrawhill's American Idioms And Phrasal Verbs