What does this phrase mean: "on purpose"?

[shwarz]

Test No. incompl/inter-94 "Letter of Complaint", question 6

I stayed in all day on ......... as you had informed me that you could not be sure at what time of day the washing machine would arrive.

(a) intention
(b) regard
(c) purpose
(d) plan

Test No. incompl/inter-94 "Letter of Complaint", answer 6

I stayed in all day on purpose as you had informed me that you could not be sure at what time of day the washing machine would arrive.

Correct answer: (c) purpose

Your answer was: incorrect
I stayed in all day on intention as you had informed me that you could not be sure at what time of day the washing machine would arrive.
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Hi! Could you tell me exactly
what does it mean
I stayed in all day on purpose
Why do you use word Purpose
Thank you very much

[Alan]

This is a set expression and means did this for a specific reason.
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English as a Second Language
You can read my ESL story Guy Fawkes Night for You