Likewise or similarly in other lexica; see the references at Photius iota125 Theodoridis. If the headword is intended to be future tense of i(ma/ssw, it should have a single sigma, but lexica (including the Suda) tended to be indifferent to single or double letters. In any event, it is probably quoted from Homer, Iliad 15.17, where Zeus threatens to do this to Hera.
For another form of this verb, see iota 335.