'plainly'(Modern English )
Adv. use of the ME adj. witter, see weterly (adv.) for full discussion.
Proposed ON Etymon (OIcel representative)
vitr (adj.) ‘wise’, cp. vitrliga (adv.) ‘wisely, with wisdom’
(ONP vitr (adj.), cp. vitrliga (adv.))
Other Scandinavian Reflexes
Far vitur, Ice vitur, ODan witær, OSw viter, Sw vitter
cp. witran (v.) 'verare, to speak the truth'; wita (n.) ‘wise man’, witan (v.) ‘to know’
Phonological and morphological markers
The adj. is first attested in late OE (see Hofmann §386, Peters 98, SPS); it is attested in the SWM in early ME (LB, AW), but in the later period it is mainly N/EM. This is MED's only citation of the adv. in this form, but weterly (adv.) is relatively widespread in early ME, though thereafter esp. frequent in alliterative texts (inc. PP).
Occurrences in the Gersum Corpus
OED identifies the instance at Cl 1552 with the v. instead (see witter v.1), but the line makes better sense if it is taken as an adv. (as all others do).