1. [= upa + kaccha1 + ka] like an enclosure adj. in the form of a hollow or a shelter Ja.i.158
  2. [= upa + kacchā2 + ka] like the armpit, a hollow usually the armpit, but occasionally it seems to be applied to the hip or waist Vin.iii.39; Vin.iv.260 (pudendum muliebre); Mil.293; Ja.v.437 (= kaccha2).

upa + kacchā + ka, cp. Sk. upakakṣa in diff. meaning