If the outside tap is what we call a frost-free sill-cock; it will have a couple of notches in the back plate which touches the wall. A couple of plastic anchors drilled into the wall with a screw in each one will anchor it.
Something tells me this one is not like that though so we have to do, some unconventional thinking; get a tub of plumber's putty (1/2 liter). Pull the faucet from the outside make a fat worm of putty into a ring the size of the pipe behind the wall plate, push the plate into it or better yet go inside and pull the pipe back to the inside and brace it there, put another worm around the pipe on the inside and use something like a popsicle stick to tamp it in around the pipe. Leave it like this for a week or so to allow the plumber's putty to cure some (it may take a year or so to set up completely.)