indeclinable emphatic (adversative) part.

1. of affirmation & emphasis: but, indeed, rather Ja.iii.499 = Ja.vi.443; Ja.v.180; Ja.vi.552
2. as 2^nd component of a disjunctive question, mostly in corresponsion udāhu… ādu (= kiṃ… udāhu Snp-a.350), viz. is it so… or Thag.1274 = Snp.354; Pv.iv.3#17 = Dhp-a.i.31; Ja.v.384 Ja.vi.382; without udāhu at Ja.v.460 (adu). The close connection with udāhu suggests an expl^n. of ādu as a somehow distorted abbreviation of udāhu.

see also adu