Lust
is,

A longing; especially for something that is forbidden.

'Lust of the flesh' would then be a longing
or eager desire for the forbidden
things of the flesh.
(See Galatians
5:19-21
)   

This I say then, Walk in the Spirit,
and ye shall not fulfill the lust of the flesh.  For the flesh lusteth against the Spirit, and the Spirit against the flesh:
and these are contrary the one to the other: so that ye cannot do the things that ye would. But if ye be led of the Spirit, ye are not under the law.

Galatians 5:   
16-18