Seg = 539. 539 is 1337 in hexadecimal form. Too many people were found to be using 1337, therefor the truely 1337 have turned towards their base 16 saviour, hexadecimal numbers!
Seg, or 539, can be used instead of the word 'Sex', as well as meaning 1337. eg...
-I'm so 539sy (I'm so sexy)
-OMG 539 (Oh My God, 1337)
-539z0r HaxXz0r (1337 hacker)