To be indebted to someone.
To constrain someone by force or by social, moral or legal means.
To do (someone) a service or favour (hence, originally, creating an obligation).
To put off; to postpone.
To decelerate; to slow down.
To keep delaying; to continue to hinder; to prevent from progressing.
Retardation; delay.
A slowing down of the tempo; a ritardando.
A person or being who is extremely stupid or slow to learn.