Same or similar entry in other lexica; see the references at Photius lambda58 Theodoridis. To judge by the gloss, the primary headword is neuter nominative/accusative singular (rather than masculine accusative singular) of this irregular superlative of lalos, and as such is probably quoted from Menander's Heiress: a)=r' e)sti\ pa/ntwn a)grupni/a lali/staton (fr. 164 Kock = 152 Koerte = 129 Kassel-Austin).
The masculine form is attested in Euripides, Cyclops 315, though not there in the favourable sense glossed here (for which see rather e.g. Lucian, Gallus 2: genitive).
For the comparative degree see lambda 79.