Etymology

ME tidi was probably formed on the n. continuing OE tīd ‘time’ (cp. OIcel tíð, OFris, OS tīd, OHG zīt < PGmc *tīðiz), with input from the ON adj. formed on the same root, cp. OIcel tíðr 'frequent, usual, customary, noted, famous, beloved, eager' accounting for the metaphorical sense extensions (MED's sense (2)) found in ME, incl. WA (2367) tidiest 'best, ablest'. WA attests variant spellings suggesting a final fricative, which might alternatively indicate that this word fell together with an adj. derived directly from the ON: cp. tethe (663), tethiest (2198), tithiest (2784). Variants with initial fricative, however, are harder to explain: cp. D MS thethiest next to tethiest (2198) and A MS theþeest next to D MS tithiest (2371).

PGmc Ancestor

?*tīð-

Proposed ON Etymon (OIcel representative)

tíðr 'frequent, usual, customary, noted, famous, beloved, eager'
(ONP tíðr (2) (adj.))

Other Scandinavian Reflexes

Far tíður, Icel tíður, Norw tid, Dan tith

OE Cognate

cp. tīd (n.) ‘time’

Phonological and morphological markers

[ON fricative /ð/ < PGmc */ð/] (may not be applicable)

Summary category

CC2

(C3)

Attestation

MED cites relevant senses from a handful of texts beginning with a1375 WPal. (KC 13) and including DT, PP and Lydgate. The variant spellings attested in WA, however, lead the OED to create an entry for an adj. tethy, which it cites only from this text. 

Occurrences in the Gersum Corpus

WA 663, 2198, *2371 etc.

Skeat WA reads teche at 663, but all others read tethe, which gives a much better sense (see TPD 663n).  At WA 2371 the A MS has <theyest>, which Skeat emends to *threyest, but TPD prefer *theþeest (cp. D tithiest) (and see TPD 2496n).

Bibliography

MED tīdī (adj.) , OED tethy (adj.) , OED tidy (adj., n. and adv.) , HTOED , de Vries tiðr (2), Mag. tíð (tíður), Orel *tīđiz, Kroonen *tīdi-