Ap (áp-) is the Vedic Sanskrit term for "water", which in Classical Sanskrit only occurs in the plural āpas (sometimes re-analysed as a thematic singular, āpa-), whence Hindi āp.
In archaic ablauting contractions, the laryngeal of the PIE root remains visible in Vedic Sanskrit, e.g. pratīpa- "against the current", from *proti-hxp-o-.
In the oldest of these, 7.49, the waters are connected with the drought of Indra.
In Vedic astrology, the female deity Apah is the presiding deity of the Purva Ashadha asterism, meaning "first of the aṣāḍhā", with aṣāḍhā "the invincible one" being the name of the greater constellation.
In Hinduism, it is also the name of the deva Varuna a personification of water, one of the Vasus in most later Puranic lists.