Same or similar entry in other lexica; see the references at Photius lambda 84 Theodoridis. The headword phrase combines the present participle (masculine nominative singular) of the verb e)ntei/nw with the adverb la/c (formed like pu/c "with the fist" and gnu/c "with the knee"). Though it shows broad similarities with phrases in Homer, notably la\c podi\ kinh/sas in Iliad 10.159 and Odyssey 15.45 (lambda 111), its actual origin appears to be patristic Commentary on the Psalms (Eusebius, Origen, Cyril of Alexandria).
[1] This glossing phrase sounds epic/poetic but is in fact attested only at Diodorus Siculus 4.59.4 (of the murderous methods of the mythological Skeiron: sigma 568).
[2] Literally, by the fingers of the foot. (But lax is nowhere else attested as a sound.)