About 「同じ」 {onaji} and 「同じく」 {onajiku}

Question

As far as I know, 「同じ」 {onaji} is not a 形容詞 {keiyoushi} (-i adjective) so how does it become 「同じく」 {onajiku}? Or does {onajiku} not come from {onaji}?

Also, are there any other non i-adjectives that have -ku counterparts (regardless of the answer for above)?

Answer 1

同じ　is mainly used as 形容動詞 (Adjectival noun) like,

彼と父親の趣味は同じだ (He and his father's hobby are the same.)

but it also can be used as adjective (形容詞) with an irregularity that the -i ending is dropped. For example,

彼は父親と同じ趣味をもっている。 (He has the same hobby with his father)

But 同じく has special usage that mean そして同じように, for example.

A-さん：　私は～部所属の～と申します。　(I am from ~ department and my name is ~)

B-さん：　同じく～部所属の～と言います。 (I am also from ~ department and I am ... )

Answer 2

In fact, 同じく does come from 同じ. The key here is to look at the historical form of 同じ: it was originally a regular adjective, following the pattern of the shiku-type adjectives (シク活用形容詞). [1] If it followed the normal development of shiku-type adjectives, it would have become 同じい, which apparently does occur, if rarely. [2]

There are some other -ku adverbs whose adjectivial counterparts are (essentially) extinct in modern Japanese. For example, 如く (as in 〜が如く) was originally derived from the adjective 如し, and similarly べく (as in なるべく) from 可し. (The latter is not a perhaps good example, as some of its other forms survive, e.g. べき, べからず etc.)

There is a (probably) unrelated morpheme -(a)ku which nominalises verbs and adjectives. Examples include 曰く←言う and 思惑←思う. It is unproductive in modern Japanese.