DSL - SND1   IMPHM, int. Also im(p)him, imp'm, euphm, imph. An exclam., used with a variety of intonation to indicate attentiveness, decided or reluctant assent, sarcastic agreement, hesitation or the like: well?, really, yes, hm! Gen.Sc. [m&mringbl;m, &mringbl;m, m&mringbl;]
    *ne.Sc. 1844 D. Grant Lays 87:
    Dinna say a word at present, Tak' an ook to think, or mair, Gin it's ``imphim'' Littlefirlot Sall mak' a' the rest his care.
    *Rnf. 1870 J. Nicholson Idylls 50:
    That auld Scotch word ``Imph-m'', ye ken it means A-y-e!
    *Ayr. 1901 G. Douglas Green Shutters xii.:
    ``Imphm; imphm; imphm; there might be something in that!'' nodding his head and stroking his moustache, as he uttered each meditative ``imphm.''
    *Dmf. 1915 J. L. Waugh Betty Grier 27:
    ``Yes, sir,'' ``No, sir,'' ``Ay, ay; imphm!'' have so far been the sum-total of his contribution to the conversation.
    *Rxb. 1921 Kelso Chron. (18 Feb.) 2:
    Aye, aye --- mebbe --- euphm --- he may be a' richt, but oh, thae bruits o' offechals.