man-command-line() {
    [[ -z $BUFFER ]] && zle up-history
    if [[ $BUFFER == man\ * ]]; then
        LBUFFER="${LBUFFER#man }"
    else
        LBUFFER="man $LBUFFER"
    fi
}
zle -N man-command-line

# Defined shortcut keys: [F1][F1]
bindkey "OPOP" man-command-line