Etymology

A compound n. formed on the OE num. twelf ‘twelve’ (cp. Go twalif, OIcel tolf, OFris twelef, OS twelif, OHG zwelif) + mōnað ‘month’ (cp. Go mēnoþs, OIcel mánaðr, mánuðr, OFris mōnath, OS mānuth, OHG mānōd). While the phrase twelf mōnað ‘twelve months’ appears a number of times in OE from the 9c., it is not used as a compound n. in the sg. in OE; the nearest analogue is the ON compound represented by OIcel tolf-mánuðr ‘a twelve-month’. Loan-translation from ON is therefore is possible (as implied perhaps by GDS, MED), but a native development is most likely.

PGmc Ancestor

*twalifa- + *mēnōþ-

Proposed ON Etymon (OIcel representative)

tolf-mánuðr ‘a twelve-month’
(ONP tolf-mánuðr (sb.))

Other Scandinavian Reflexes

OE Cognate

twelf ‘twelve’ + mōnað ‘month’; cp. twelf mōnað ‘twelve months’ 

Phonological and morphological markers

Summary category

CCC4

Attestation

Clear uses of the compound as sg. n. are attested in MED from the early 13c. (inc. AW.A), and are widespread thereafter.

Occurrences in the Gersum Corpus

Gaw 298, 383, 2243; WA 5429

Bibliography

MED twelve-mōnth (n.) , OED twelvemonth (n.) , HTOED , Dance twelmonyth, de Vries tolf; mánaðr, Mag. tólf; mánuður, Bj-L tolv; måned, Bammesberger 216, Orel *twalifa; *mēnōþz, Kroonen *twalifa-; *mēnōþ-, AEW twelf; mōnað (2)