SIP VS SIP AT

I sense no difference in meaning. The transitive form is more common.

aramahosi"He sipped the drink" vs "He sipped at the drink"

This is an example, though perhaps not the most central case, of what Levin (English Verb Classes and Alternations) calls a conative alternation. She comments thus on verbs with this property:

The use of the verb in the intransitive variant describes an "at

So "She unintentionally stabbed at him with a knife" isn't an acceptable English?

aramahosiSo "She unintentionally stabbed at him with a knife" isn't an acceptable English?

I don't know how you can come to that conclusion.

CalifJimThe use of the verb in the intransitive variant describes an "attempted" action without specifying whether the action was actually carried out. The conative alternation seems to be found with verbs whose meaning includes notions of both contact and motion.

So, I pulled the rope means that my act of pulling was followed by a movement of the rope,

Henry74Sorry to barge in like this.

Don't worry about it.

Henry74So, I pulled the rope means that my act of pulling was followed by a movement of the rope, whereas I pulled on the rope doesn't say if the rope actually moved or not.

That's the general idea, yes. Still, you can contradict the usual meaning with another