According to Online Etymology Dictionary, the word is an alteration of the 1940s term "nert" (meaning "stupid or crazy person"), which is in itself an alteration of "nut" (nutcase). The term was popularized in the 1970s by its heavy use in the sitcom Happy Days.

Who came up with nerd?

Seuss invented the word 'nerd,' and 42 other author-invented words.

What word came before nerd?

Before “geek”, “nerd”, “dork”, etc, the proper terms for these same ragamuffins were “Dewdroppers”, “Waldos”, and “Slackers”. Other common old slang words that were somewhat similar in meaning: pantywaist, oil can, drip, stinkeroo, mullet, roach, schnookle, kook, dimp, dorf, squid, auger, square, Joe Zilch, and dudd.