How to bindkey alt+key in OSX

M-b and M-f should be bound to backward-word and forward-word by default:

$ zsh
$ bindkey | grep -- -word  
"^W" backward-kill-word
"^X*" expand-word
"^[^H" backward-kill-word
"^[^_" copy-prev-word
"^[\$" spell-word
"^[." insert-last-word
"^[B" backward-word
"^[C" capitalize-word
"^[D" kill-word
"^[F" forward-word
"^[L" down-case-word
"^[S" spell-word
"^[T" transpose-words
"^[U" up-case-word
"^[_" insert-last-word
"^[b" backward-word
"^[c" capitalize-word
"^[d" kill-word
"^[f" forward-word
"^[l" down-case-word
"^[s" spell-word
"^[t" transpose-words
"^[u" up-case-word
"^[^?" backward-kill-word

You just have to change the meta key: