un ‧ block /ʌnˈblɒk $ -ˈblɑːk/ BrE AmE verb [transitive]
[ Word Family: verb : ↑ block , ↑ unblock , ↑ blockade ; noun : ↑ block , ↑ blockage , ↑ blockade ; adjective : blocked, unblocked]
if you unblock a pipe, you remove something that is blocking it
unblock a toilet/drain/chimney etc