Etymology

Most authorities connect the vocalism of ME sene with that of OE Angl. gesēne ‘seen, visible, evident, plain’, which is often taken to be a dialect variant of WS gesīene, gesȳne, and thus the direct cognate of the ON word represented by OIcel sýnn (‘visible, clear, evident; sightly’) and of Go ana-siuns, OHG -siuno, MLG süne, MDu siene, etc., < PGmc *segwni-. But Angl. ē is not the regular reflex of i-mutated /iu/, and others have preferred to explain gesēne instead as a form of the pp. of the v. sexwan- (see further Heid. and Seebold) or as a related word showing a different Ablaut grade. OED is alone in suggesting Norse input in hastening the dropping of the prefix through the influence of the corresponding ON sýnn. 

PGmc Ancestor

*segwni- or *sexwan-

Proposed ON Etymon (OIcel representative)

sýnn ‘visible, clear, evident; sightly’
(ONP sýnn (adj.); sjá (2) (vb.))

Other Scandinavian Reflexes

Icel sýnn, sýn, Norw syn, OSw syn

OE Cognate

gesēne ‘seen, visible, evident, plain’ (WS gesīene, gesȳne)

Phonological and morphological markers

Summary category

CCC1a

Attestation

Widespread in ME as a pp.; cited five times by  MED in sense 20c (‘plain, clear, manifest’), inc. AW and Malory.

Occurrences in the Gersum Corpus

Gaw 148, 197, 239, etc.; Pe 164, 194, 787, etc.; Cl 1169; Erk 100; WA 62, 1551, 2956 etc.

The forms of this word in Gaw are apportioned variously amongst glossary/dictionary head-words. TGD puts all six instances s.v. sen(e) adj., glossing 148 as ‘plain to see’, 341 as ‘plain, clear’, and the rest as ‘seen’ (used as the pp. of the v. se).  GDS has one entry for the adj. meaning ‘visible, clear’ ( 148, 341, 468), and another for the vb. se (pp. at 239 and 475; 197 is missing). Vantuono also has a separate entry for the adj. at 148 and 341. OED cites 341 (only) under sene, i-sene (adj.); MED has 341 in its entry for the v. (sense 20), and none of the Gaw instances s.v. isēn(e (adj.). 

Bibliography

MED sēn (v.1) (sense 20) ; isēn(e (adj.) , OED see (v.) ; sene, i-sene (a.) , HTOED , HTOED , Dance sene, de Vries sýnn; sjá (2), Mag. sýnn (1); sjá (2); sénn, Bj-L. se; syn, Seebold sehw-a-, Heid. (-)segwni-, Orel *seʒwniz (II); *sexwanan, Kroonen *seuni-; *sehwan-, AEW ge-sīene