adj. nt. “putting under”, i.e.

1. a pillow, cushion DN.i.7; SN.ii.267 = Mil.366 (kaḷingar˚); SN.iii.145; AN.i.137, AN.i.181; AN.iii.50; Ja.iv.201; Ja.v.506 (tamb˚ = ratt˚ C.).
2. imposing, giving, causing Dhp.291 (dukkh˚)

fr. upa + dhā, cp. upadahati