means : ownz = owns . how did it came out ? well , i guess it was a typo . like , someone wanted to write owns and he wrote pwns . and pwns became pwnz and so on ... pwn !!!
1)somebody has a foolproof control of something. and it is really a tighter control than that depicted by ownz. This controled party has virtually no chance of escape left for him.
Usually used in the hacker subculture when machines becomes controled by a hacker
2)In playing pranks or jokes, when someone is left with no defence.
To sum up, it means "total control" over someone or something
1. Conjunction of 'password-ownz'. To 'own' the master password for a system. Originated in '1337-speak'. After an attacker has successfully taken complete control of a system, he may claim that he "pwnz" it rather than the legitimate owner. Indicates a superior level of control in relation to 'ownz'
2. To completely subjugate a competitor or opponent, to the point of no recourse.