Etymology

Clearly related to hap, though the nature of the derivation (and the origins of the suffix -n) are somewhat opaque. It is usually explained as a loan of the formally similar ON adj., cp. OIcel heppinn ‘lucky’ (a derivation on happ with i-mutation), with the vocalism replaced by that of ME hap (thus MED, TGD). The OE adj. gehæp could account formally for the root syllable of hapnest; but the senses recorded for it are a markedly less good fit, and hapnest clearly owes at least its semantic development to the group of ME words in hap- normally derived from ON and relating to (good) fortune, happiness, etc. 

PGmc Ancestor

*xap(p)- 

Proposed ON Etymon (OIcel representative)

heppinn ‘lucky’
(ONP heppinn (adj.))

Other Scandinavian Reflexes

Far heppin, Icel heppinn, Norw heppen, Sw dial vanhäppen

OE Cognate

cp. gehæp and gehæplic (adj.) ‘fit, convenient, opportune'

Phonological and morphological markers

Summary category

CC1c

(CC3)

Attestation

MED and OED only record attestations from a few ME texts: Gaw, Pat and St. Placidus. Cp. further vnhappen (adj.).

Occurrences in the Gersum Corpus

Gaw 56; Pat 13, 15, 17, etc.

Bibliography

MED happen (adj.) , OED3 happen (adj.) , HTOED , EDD heppen (adj.), Dance hapnest, Bj. 212–13, de Vries heppinn, Mag. heppinn