meaning of "six pack" referring to "muscles" or 'work out'

[Starr]

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #315 "Health and Nutrition Questions Test (1)", question 6

......... is a substance that is found in many drinks and foods such as coffee and chocolate bars. This substance can keep you awake for long periods of time.

(a) Gelatin
(b) Spice
(c) Caffeine

English Language Proficiency Tests, Advanced Level

ESL/EFL Test #315 "Health and Nutrition Questions Test (1)", answer 6

Caffeine is a substance that is found in many drinks and foods such as coffee and chocolate bars. This substance can keep you awake for long periods of time.

Correct answer: (c) Caffeine

Your answer was: correct
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Could you explain the meaning of "six pack" referring to "muscles" or "work out" to me please? Or give me a link to a dictionary where it is possible to find it?

Thanks in advance.

Starr

[Mister Micawber]

(bodybuilding): A highly developed set of abdominal muscles.
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Native English teacher at Mister Micawber's