Etymology

Always derived from ON, cp. the OIcel pret.-pres. aux. v. munu (variant monu in older mss) ‘will, shall, may, must’, pres. 1/3 sg. mun (variant mon), later man. This sense seems to have developed as a usage of the common Gmc v. represented by OIcel muna ‘to mind, call to mind, remember’, cp. OE (ge)munan (pres. 1/3 sg. similarly man), Go (ga)munan, OS -man (in far-man ‘despises’). Spellings in <o>, as in Gaw, could represent /o/, reflecting the ON variant man, which in that case would be formally indistinguishable from the descendant of the pres. 1/3 sg. of OE munan, but the sense development and aux. usage make derivation from ON much more likely. The ME record is dominated by spellings in <u> or <o>, and thus it is usually assumed that the ON pres. 1/3 sg. variant mun lies behind the ME forms, with <o> spellings standing not for /o/ but /u/.

PGmc Ancestor

*munan-

Proposed ON Etymon (OIcel representative)

munu ‘will, shall, may, must’
(ONP munu (vb.))

Other Scandinavian Reflexes

Far munna, Icel munu, Norw mun, muna, muno, Dan monne, Sw månna, månde

OE Cognate

(ge)munan ‘to mind, call to mind, remember’ 

Phonological and morphological markers

Summary category

C3c

Attestation

Common in MED in texts from the N/EM, as far south as c1436 Ipswich Domesday(2) (Add 25011), and occasionally further afield (viz. c1455 Spec.Miser.(Tak 32), put by LALME in Kent (LP 5970)), but nevertheless predominantly associated with N dial usage (Thorson 69). In wide MnE dial use, esp. N and Midlands (see further OED).

Occurrences in the Gersum Corpus

Gaw 1811, 2354; WA 691, 707, 1502 etc.

Most take mon at Gaw 2354 as an auxil. v. ‘must’ (inc. TGD, GDS, AW, Vant, PS, Knigge 1885: 78, MED), but (taking their lead from the apparent reading by Madden and Morris) Emerson (1922: 408), Sisam (C14 verse and prose, his l. 286) and Jones understand it as mon ‘man’, and gloss ‘trwe mon trwe restore’ as ‘if a true man truly restore’ (Emerson) or ‘let a true man truly restore’ (Sisam; Jones ‘let a true man truly repay’), taking restore as pres. subj. sg. The D MS of WA reads <must> at 707.

Bibliography

MED monen (v.2) , OED3 mun (v.) , Dance mon, de Vries munu, Mag. munu, Seebold man, Orel *mana, Kroonen *munan-, AEW munan