I direct derivation from √LEK. Based on a Goldogrin (Sindarin's "draft") verb, laitha- "to let slip, lose, mislay, forget...", which undoubtedly survived in Sindarin with slight alterations to its sense and form, as leitha- "to release, set free". Thus, one could simply use leitha- for "to forget", but in order to avoid overburdening the verb, leg- as a neologism would be preferred. Another approach to the subject is "normalising" the Goldogrin verb as laetha-, and yet another is to combine dan with ren- as *dadhren-, which literally means "to remember back". There is no consensus on this matter, since all forms have arguments both for and against them.
* leg-なx$ verb. to forget