in England this means a person is stupid.e.g '1+4 isn't 6,don't be so thick' 'are you thick! don't put your finger there'. originally thick head,or fat head but thick sounds better. if your an american in england, calling a girl thick will not go down well.it will mean that you think she is a dumb arse.
in ENGLAND : 'mate you are so thick, 1 squared isn't 2'.
in america, ( a great country, i've been on holiday):'that girl is thick'*.
*(for the english, thick in america means not skinny but not fat- a good shape)