1640s, from Latin aqua "water" (see aqua-) on analogy of French aqueux "watery" (16c., which, however, is from Late Latin aquosus "abounding in water"). Or by analogy of Latin terreus "earthy," from terra "earth." Aqueous humor is the original use in English.
aqueous a·que·ous (ā'kwē-əs, āk'wē-)
Relating to, similar to, containing, or dissolved in water; watery.