A few people have described the bottom half of a bikini as a "slip", for example, http://www.makehumancommunity.org/cloth ... _slip.html. I'm guessing that, in some languages, the word that translates to English as "slip" has multiple meanings. In English, it's pretty specific (this is the first definition Google throws up):
a woman's loose-fitting, dress- or skirt-length undergarment, suspended by shoulder straps ( full slip ) or by an elasticized waistband ( half slip ).
A slip is typically worn under a skirt (or dress)--or rather, it was typically worn, and it's rarely seen nowadays, especially in casual clothes, though some skirts and dresses have slips sewn in (basically, just an inner layer of a different material).
I'm afraid that, despite the otherwise extensive vocabulary of English, replete with borrowings from all over the world, we haven't come up with a real name for the bottom half of a bikini, which we call, uncreatively, a "bikini bottom". There's more nuance to the use of "bikini", which can also refer to underwear (this is English--there's always nuance in meanings). If you're curious, see https://ell.stackexchange.com/questions ... i-swimsuit. However, "bikini bottom" is the one term in English that unambiguously refers to the lower part of a two-piece women's swimsuit.