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1.
1. a sure thing; a lock; goal attainment is assured; prospects are brilliant

2. all over it; about to take care of with gusto

From the tendency of thongs and other tightfitting swimwear to become wedged where the sun never shines

"In there like swimwear" is "in like Flynn" for a new millenium. Can you name a single Errol Flynn movie? But everyone knows (and loves) the Thong Song.
"Make friends with her friends and you'll be in there like swimwear."
from The Player's Rulebook
by mandingoe June 15, 2004