This word is quite often used as a female form of Metalhead
(although metalhead's can be female)
A true metalchick dresses in a similar way to a male metalhead. I.e - tight, faded jeans, band t-shirts(usually in skinny fit) some piercings or tattoos, and some jewelery. This type of Metalchick tends to dig Metallica, Machine Head, Slayer.....etc.
Some Metalchicks may dress more like girls are 'supposed' to. I.e - very tight pants(usually leather or pvc) tight/revealing tops, and big fluffy hair. This look is favoured by girls who like bands like Motley Crue, Kiss and Van Halen.