Old English gidig, variant of gydig "insane, mad, stupid," perhaps literally "possessed (by a spirit)," if it is from Proto-Germanic *gud-iga- "possessed by a god," from *gudam "god" (see god (n.)) + *-ig "possessed." Meaning "having a confused, swimming sensation" is from 1560s (compare sense evolution of dizzy). Meaning "elated" is from 1540s. Related: Giddily; giddiness.
word-forming element denoting action, quality, or state, attached to an adjective or past participle to form an abstract noun, from Old English -nes(s), from Proto-Germanic *in-assu- (cognates: Old Saxon -nissi, Middle Dutch -nisse, Dutch -nis, Old High German -nissa, German -nis, Gothic -inassus), from *-in-, originally belonging to the noun stem, + *-assu-, abstract noun suffix, probably from the same root as Latin -tudo (see -tude).
<a href="https://www.etymonline.com/word/giddiness">Etymology of giddiness by etymonline</a>
Harper, D. (n.d.). Etymology of giddiness. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/giddiness