The only free and comprehensive online etymological dictionary of the Spanish language
laja f. (Noun) "leash"

16th cent. From Latin laxa "wide," "loose." From Proto-Italic *slaksā-, masculine *slakso- 'id.' From Proto-Indo-European *sl̥ǵ-so- 'id.'

Romance: Western Vulgar Latin: Italian lasso

Italic: Latin languere "to be faint" ((s)l-n̥-ǵ-u̯- "faint")

Indo-European: Germanic: Old Norse slakr "weak," Old High German slach 'id.,' Old Saxon slak 'id.,' Old English slæc 'id.' (English slack); Hellenic: Ancient Greek λάγνος (lágnos) "voluptuous;" Indo-Iranian: Sanskrit ślakṣṇá- "slippery;" Tocharian: A slākkär "sad," B slakkare "darting"