लग्

Of uncertain origin.

Burrow derives the root from an earlier form *स्लग्न (slagna), *स्रग्न (sragna), and connects it to *स्रक (sraka, “wreath, garland”), which is usually attested as स्रज (sraja). Mayrhofer tentatively favors these hypothetical proto-forms, though seems to have rejected their connection to स्रज (sraja) by the time of writing the EWAia. He does not, however, propose a particular Indo-European root that the Sanskrit would trace back to.

One possible root that matches formally is Proto-Indo-European *(s)leh₂gʷ- (“to seize, latch onto”), whence Ancient Greek λάζομαι (lázomai, “to seize, grasp”) and Old English læċċan (whence English latch). Considering both phonetics and semantics, as well as the root's attestation in both an eastern and western branch of Indo-European, this seems to be the most plausible derivation.

Another possible root that matches formally is Proto-Indo-European *(s)leg- (“to tire out, slacken”), with semantic shift "to be limp, slack" > "to 'lazily' cling onto" (perhaps in a similar way to how dripping slime tends to not separate easily) > "to adhere". If from this root, then cognate with Latin langueō, Ancient Greek λᾰ́γνος (lágnos), and Proto-Germanic *slakaz (whence English slack). However, the semantic shift required for this is far from trivial.

Older theories tentatively connected the root to रज् (raj), रञ्ज् (rañj, “to redden, dye”); under this derivation, रज् (raj) took on a more specific sense from "to attach" > "to attach color (i.e. to dye)". This is semantically tenuous and without textual evidence. Other theories connecting the word to लक्ष् (lakṣ, “to recognize; mark”) are similarly unconvincing.

It is also worth noting that the root bears a very strong semantic similarity to Proto-Indo-European *leyǵ- (“to bind”), whence English ligand. However, the phonetics are mismatched, particularly the palatal *ǵ in the Proto-Indo-European root, and would have to be sufficiently explained, in addition to the existence of *leyǵ- being under question.

लग् • (lag)

1. to attach, stick to, cling
2. to meet
3. to pass (as of time)

• Monier Williams (1899) “लग्”, in A Sanskrit–English Dictionary, […], new edition, Oxford: At the Clarendon Press, →OCLC, page 893/2.
• William Dwight Whitney, 1885, The Roots, Verb-forms, and Primary Derivatives of the Sanskrit Language, Leipzig: Breitkopf and Härtel, page 144
• Mayrhofer, Manfred (1996) Etymologisches Wörterbuch des Altindoarischen [Etymological Dictionary of Old Indo-Aryan]‎ (in German), volume 2, Heidelberg: Carl Winter Universitätsverlag, page 473
• Mayrhofer, Manfred (1976) Kurzgefasstes Etymologisches Wörterbuch des Altindischen [A Concise Etymological Sanskrit Dictionary] (in German), volume 3, Heidelberg: Carl Winter Universitätsverlag, page 84