Interesting etymology for this one. OED explains (although as Byron wrote of Coleridge: “Explaining metaphysics to the nation/I wish he would explain his explanation").
Etymology: Old English phrase þý lǽs þe , lit. ‘whereby less’ = Latin quōminus (þý instrumental of the demonstrative and relative pronoun + lǽs less adj. + þe relative particle). In Middle English the first word of the phrase was dropped, and les þe became les te in accordance with the general rule that þ after s changed into t.
It’s the ‘instrumental of the demonstrative and relative pronoun’ and ‘relative particle’ that I’m unsure of. Isn’t the instrumental the ablative? Is the pronoun thou? Does the Old English phrase translate as ‘thy less thee’?
OK, enough obtuseness demonstrated!