IMHO, ประสะ is from Pali 'passa ปัสสะ' : side; flank. (m.; nt.);  as a verb it means 'be located at the sides of something or somebody ' .

ประสะนอแรด = medicine with rhino horn (on the side)

[later it was found that rhino horn was not among its ingredients, to avoid misrepresentation (by law), the name is changed to ประสะบอแรด in a play of word บอ: ว. เกือบบ้า, ใกล้จะเป็นบ้า, ครึ่งบ้าครึ่งดี and บ่อ (chinese) ไม่.]

;-)