Both headword and gloss are dative plural adjectives. The form of the headword is ambiguous in terms of gender, but the gloss is feminine; and usage of this adjective suggests that the associated or implicit noun is probably morfh/ ('shape').
This item appears in other lexica too but is not independently attested, so the source of the quoted headword is indeterminable.
For another compound from brotos (beta 555) see beta 553 and beta 554.