The headword is the present active infinitive of the contract verb plhmmele/w, I make a false musical note, but, as LSJ s.v. notes, in usage it has the metaphorical sense of offending, erring, or being disorderly. The entry is perhaps generated from cognates of the headword attested in the quotations given; if not that, its origin is unclear.
[1] The glosses are present active infinitives of the verbs a)takte/w (I am undisciplined, indecent), u(bri/zw (I outrage, insult), and r(a|qume/w (I neglect, am remiss); see generally LSJ s.vv. The headword is identically glossed in Timaeus' Platonic Lexicon and Photius' Lexicon (pi945 Theodoridis).
[2] Plato, Lysis 222B (web address 1): Socrates stipulates that any friend is useful in some way.
[3] Plato, Laws 9.859E (web address 2, with an a)/n imported from part of the sentence not quoted): the Athenian contends, against some push-back from Clinias the Cretan, that it is not wrong to say that physically ugly men might be morally beautiful.
[4] Quotation unidentifiable.
[5] Theophylact Simocatta, Histories 2.15.7. It describes an incident in 587 CE where a military column marching under the command of the Byzantine general Komentiolos (Comentiolus, of Thracian origin, d. 602, kappa 1990, alpha 3649; PLRE, pp. 321-5 and Whitby, p. 15-6) was disrupted by a draft animal having lost its load; see Whitby and Whitby, pp. 64-5.