The ASL sign for "BIKINI" meaning, a style of women's bathing suits, is commonly produced by signing BRA or BRA + UNDERWEAR.
BRA + UNDERWEAR:

You can also add SWIM before signing BRA or BRA + UNDERWEAR to make it clear that you are referring to the bathing suit, not women's undergarments.
SWIM + BRA + UNDERWEAR:

Notes:
See: SUN
See: SWIM
See: POOL
See: SUMMER
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