1
general::
Withdraw from participation in a group such as a school, club, or game; also, withdraw from society owing to disillusionment. For example, He couldn't afford the membership dues and had to drop out, or She planned to drop out from college for a year. [Late 1800s]
American Heritage Idioms
2
general::
Phrase(s): drop out (of something)
1. Lit. to fall out of something. • One by one, the skydivers dropped out of the plane. • The marshmallows dropped out of the bag. 2. Lit. or Fig. [for the bottom of something] to break loose and drop. • The bottom dropped out of the box, spilling everything everywhere. • The bottom dropped out of the stock market, and we lost a lot of money. 3. Fig. [for someone] to resign from or cease being a member of something; [for someone] to leave school. • Sally dropped out of school for some unknown reason. • But why did she drop out?
McGrawhill's American Idioms And Phrasal Verbs