Usage: 2 forms in 2 verses
Form Translated As     In Verse(s)
πενθικοίσ mourning LXX.Exod.33.4
πενθικά mournful LXX.2Sam.14.2

Liddell-Scott-Jones

πενθικός (penthikos) mournful G:A

πενθικός-ῐκός, , όν,

of or for mourning, mournful, ὀδυρμοί [Refs 1st c.AD+]; ἐν πενθικόςικοῖς (i.e. ἐσθήμασι) [LXX] adverb -κῶς, ἔχειν τινός to be in mourning for a person, [Refs 5th c.BC+]

LSJ Scripture Refs: LXX.Exod.33.4