Etymology

Formed on semly (adj.).

PGmc Ancestor

*sōmi-

Proposed ON Etymon (OIcel representative)

sœmr ‘becoming, fit’; cp. sœmiligr ‘becoming’ 
(ONP sǿmi-ligr (adj.), sǿm-ligr (adj.))

Other Scandinavian Reflexes

OE Cognate

cp. gesōm ‘unanimous, peaceable, friendly’

Phonological and morphological markers

Summary category

C1a

(C3)

Attestation

MED has widespread attestations from the 14c onwards. 

Occurrences in the Gersum Corpus

WA *99, *4499

At WA 99 the MS reads <vmsemely>, but is always emended.  TPD introduce another instance in WA by emending MS <oure assemy> at 4449 to * ʒoure vnsemely (Skeat WA suggests something similar in his glossary), which restores the sense and alliteration of the line.

Bibliography

MED unsẹ̄melī (adj.) , OED unseemly (adj.) , HTOED ; see further semly (adj.) and seme (adj.).