どこ

Etymology 1
→ →  →  →

Shift from earlier and, ultimately from -derived  of the same meaning; by analogy of pronouns  and  having two morae in length, therefore the drop of the initial i-.

Compare the shift of →.

Pronoun

 * 1)  where..., which [ noun]...
 * 2)  which [ collective noun]...
 * 3)   anywhere, everywhere, somewhere
 * 1)  which [ collective noun]...
 * 2)   anywhere, everywhere, somewhere
 * 1)  which [ collective noun]...
 * 2)   anywhere, everywhere, somewhere
 * 1)   anywhere, everywhere, somewhere

Etymology 2
First cited to a text in 1790.

The suffix senses are derived from the form of, a colloquial stem indicating a circumstance, moment, place, point, and many others.

Particle

 * : the extent of, the place of, the time of

Suffix

 * 1)   befitting..., deserving..., worthy of...
 * 2)  one's family, home, or household