From Wiktionary, the free dictionary

From Middle English depnesse, from Old English dēopnes (“deepness; depth; an abyss”), from Proto-West Germanic *deupnassī, equivalent to deep +‎ -ness. Cognate with German Low German Deepnis (“deepness, depth”).

• (UK) IPA^(key): /ˈdiːpnəs/
• Rhymes: -iːpnəs

deepness (usually uncountable, plural deepnesses)

1. The state or quality of being deep (either physically or metaphorically)

   Synonyms: depth, profoundness, profundity

   Antonym: shallowness

   Coordinate term: pseudoprofundity