1
general::
Phrase(s): in case (something happens)
in the event that something takes place. • She carries an umbrella in case it rains. • I have some aspirin in my office in case I get a headache. • He keeps a fire extinguisher in his car, just in case.
McGrawhill's American Idioms And Phrasal Verbs
2
general::
1. Also, just in case. If it should happen that. For example, In case he doesn't show up, we have a backup speaker. The variant Also is used without a following clause to mean simply "as a precaution," as in I took an umbrella just in case. [c. 1400] 2. in case of;in the event of. If there should happen to be. For example, Here is a number to call in case of an emergency, or In the event of a power failure, we'll have to shift our plans. Similarly, in that case means "if that should happen," as in You're alone in the store? In that case I'll bring your lunch. The first usage dates from the early 1700s, the second (with event) from about 1600, and the third from the mid-1800s. Also see IN ANY CASE; IN NO CASE; IN THE CASE OF.
American Heritage Idioms