v. (pres. 3 sg.)
(1) ‘coasts, passes by the side of’; (2) ‘gives (full bright) quality to’; (3) ‘drives’(Modern English coast)
Derivation from ON and emendation of the MS reading of the word in Gaw (‘In rede rudede vpon rak rises þe sunne, / And ful clere costez þe clowdes of þe welkyn’) have both been proposed, but the simpler solution is the more likely: (1) Most editors (since Madden) accept the MS reading and identify it with ME cost(ei)en (PDE coast) in MED’s sense (1a), viz. ‘To go along a border, sail along the coast; pass along the border of (sth.), go alongside (sth.), skirt’, derived from AN, OFr costeier (costier) ‘to skirt, go along the side of, pass along’ (etc.)(< a late Lat *costicāre formed on Lat costa ‘rib, side, coast’); ME and subsequent forms without -ei- probably show influence from the related n. ME coste (PDE coast) < AN, OFr coste (thus OED, MED, Knott 1915: 106, Onions 1923: 285-6, TGD, etc.). (2). Wright (1936: 314) prefers derivation from the ME n. cost 'nature, quality (etc.)' < ON, cp. OIcel kostr 'condition, choice, opportunity (etc.)' (see cost) on the grounds of sense, taking 'ful clere costez' to mean 'gives full bright quality to' hence 'brightly colors, makes radiant'. However such a v. would be otherwise unknown in English and her objection to the meaning 'coasts' seems unnecessary. (3) The main alternative (suggested by Morris, followed by M(G), GDS etc.) is to emend MS <costez> to <c[a]stez> and identify it with ME casten (PDE cast) (cp. OIcel kasta ‘to cast, throw’, apparently < PGmc *kastō(j)an-; see kest (v.)), presumably with a meaning like MED’s sense (10), ‘to drive out, expel, banish’, i.e. the sun ‘drives’ the clouds from the sky.
(2) *kustiz or *kustuz; (3) *kastō(j)an-
Proposed ON Etymon (OIcel representative)
(2) kostr ‘condition, choice, opportunity (etc.)’; (3) kasta ‘to cast, throw’
(ONP (2) kostr (1) (sb.); (3) kasta (vb.))
Other Scandinavian Reflexes
(2) Far kostur, Icel kostur, Norw kost, ODan kost, OSw koster; (3) Far kasta, Icel kasta, Norw kasta, Dan kaste, Sw kasta
Phonological and morphological markers
[absence of palatalization of */k/] (may not be applicable)
(1) ME costen, costeien is fairly widespread in late ME, from (c1384) WBible(1) (Roy 1.B.6) onwards. (2) A v. costen formed on ME cost ‘nature, quality’ (etc.) would be a hapax legomenon. The n. occurs several times in late OE, and is widespread in early ME, but by the later period mainly N/E and alliterative. (3) The v. casten ‘to throw’ (etc.) is widespread from early ME.
Occurrences in the Gersum Corpus
MED cōsteien (v.) , OED coast (v.) , HTOED , Dance costez; (1) OED coast (n.) , MED cō̆ste (n.) , AND coste (1); costeier (1), FEW cǒsta; (2) MED cost (n.1) , OED cost (n.1) (cost (n.2) ), Bj. 247, de Vries kostr (1), Mag. kostur (1), Bamm. 140, 157, Orel *kustiz; *kustuz, Kroonen *kusti-; *kustu-, AEW cost (1), DOE cost n.2; cyst; (3) MED casten (v.) , OED cast (v. ), Bj. 142, 215, de Vries kasta, Mag. kasta (1)