An Etymological Dictionary of the German Language/Annotated/blecken
blecken, verb ‘to show one's teeth, grin,’ from Middle High German blęcken, ‘to become visible, show,’ Old High German blęcchen (Gothic *blakjan). Factitive of a Gothic *blikan, which, according to the law of the permutation of consonants, is cognate with Greek φλέγω, ‘to burn, shine’ (compare φλογ- in φλόξ, ‘flame’), Latin flagro, ‘to burn,’ and the Sanscrit root bhrâj), ‘to shine.’ Old High German blęcchen also means ‘to lighten, gleam, shine forth,’ For further details see Blitz.