Pacman is currently in use, please wait.
When an update gets interrupted in some way, that is, pacman/yaourt cannot finish its job, pacman may be locked for further use.
$ yaourt -Syu ==> Pacman is currently in use, please wait.
You can unlock pacman by removing
$ rm /var/lib/pacman/db.lck
And pacman/yaourt will be working again!