Think of the word entire as referring to all of anything. Now think of that "all" as being whole — not broken or lacking in any way. Remember that, and you will have the entire meaning of the word.
Entire is based on the Latin integrum, which came from the prefix in-, "not," and tangere, "to touch," and thus means "untouched." From there it developed into the 14th-century Old French entier, meaning "unbroken or complete," and then, through Middle English, into our modern word entire. One use of the word you don't see every day is to describe an uncastrated horse — an "entire horse" is a stallion. Gives a pretty interesting picture of the word, doesn't it?