Literally, अग्नि = अंग् + नि, नलोपश्च। And the root verb अंग् means "to revolve". The meaning of the suffix नि , I couldn't find anywhere, and then suddenly we start associating अग्नि with the sacrificial fire. Is it somewhat biased by tradition/convention? Maybe the meaning of the suffix has to do something here. Otherwise at certain instances in the Vedas, the meaning of verse gets too awful ( like in Rv. 2|1|1, Vaj. 11|27, त्वमग्ने ... द्भ्यः ... जायसे शुचिः। Agni being born of water - somewhat contradictory.)
Pardon me, but what I am actually asking is the "how-to" of assigning meanings to sounds/dhatus/pratyayas in sanskrit, which was perhaps known to the niruktakars at Yask's time, but now is (maybe, maybe not) lost.