[§ 432]. The adverbs of deflection (of the chief importance in etymology) may be arranged after a variety of principles. I. According to the part of speech from whence they originate. This is often an adjective, often a substantive, at times a pronoun, occasionally a preposition, rarely a verb. II. According to the part of the inflection from whence they originate. This is often an ablative case, often a neuter accusative, often a dative, occasionally a genitive.
The following notices are miscellaneous rather than systematic.
Else, unawares, eftsoons.—These are the genitive forms of adjectives. By rights is a word of the same sort.
Once, twice, thrice.—These are the genitive forms of numerals.
Needs (as in needs must go) is the genitive case of a substantive.
Seldom.—The old dative (singular or plural) of the adjective seld.
Whilom.—The dative (singular or plural) of the substantive while.
Little, less, well.—Neuter accusatives of adjectives. Bright, in the sun shines bright, is a word of the same class. The
neuter accusative is a common source of adverbs in all tongues.
Athwart.—A neuter accusative, and a word exhibiting the Norse neuter in -t.