An entry not literally excerpted from the Lexicon Ambrosianum: Ambrosianus B12 sup. fol. 76v has ἤγγυα: ἐπετίθει, ἐβεβαίου , “he/she gave as a pledge: he/she offered, he/she guaranteed”. Photius, Lexicon, eta19 (Theodoridis) and Herennius Philo, De diversis verborum significationibus (Palmieri) eta88 (where perhaps the Photian entry is taken from) have: ἤγγυα: ὡμολόγει δωσειν , “he/she agreed to give”.
ἤγγυα is imperfect indicative active, third person singular, of ἐγγυάω (epsilon 169), a verb commonly used in the classical period. In this precise form it occurs twice in Isaeus (3.45 & 70) and twice in Demosthenes (41.6 & 16).