The obsolete Dutch word poest is retained in at least the dialect of Drenthe and can mean there (see dialect dictionary of Drents (Dutch province language))
1. pimple (modern Dutch: de puist)
2. breath
3. illness: breathlessness
4. small bridge of which the middle part can be taken out
5. wooden board that can be placed over a stream
6. blowpipe (1m long) for the fireplace.
If we may extrapolate these dialect meanings to seventeenth century NY Dutch language, then common sense would suggest meaning 4: Poestenkil = Creek with the little bridge with the movable middle, or 5. Creek with the board. Perhaps there are old pictures of a bridge over the Poestenkil ? Alternatively, there might have been a Dutch settler by the (nick)name of Poest (perhaps with a pimple?), so that the name would mean say "Mr. Pimple's Creek".
Neither does the extensive multivolume Woordenboek der Nederlandse Taal WNL give the meaning "froth" or "bubbly" for poest, but is in line with the meanings given above (offers also: cowshed, piece of wood and the verb poesten (blow, brag, kiss, fight, what not! .... online results in Dutch to add to the confusion..).
mountidatroy.org
Gobert, T.A.; Cara Manley; Richard Montena (April 25, 2007). "Natural Features of Mount Ida"(PDF). mountidatroy.org. Retrieved April 23, 2009.
