Q:

"I can't help it." Why help?

Why do we use the verb help in sentences such as the following to mean something like refrain from?

I try not to eat junk, but I can't help it.
I couldn't help laughing.
I can't help but admire her bravery.

A:

Two of the OED’s definitions of help are relevant. Definition 11a is:

To remedy, obviate, prevent, cause to be otherwise. (With can, cannot, or
  some equivalent.) In earlier use usually in passive ‘it cannot be
  helped’, later in active with personal subject ‘I cannot help it’ = I cannot do anything to remedy or prevent it.

Definition 11b is:

To prevent oneself from, avoid, refrain from, forbear; to do otherwise
  than. (With can, cannot.)

It is true that in both senses help is often used with a negative word such as no, scarcely or hardly, but that is not always the case. For one thing, it can be used in a question, as in Trollope’s ‘How can I help it that I am not a man and able to work for my bread?’ It can also occur in an if clause following a negative, as it did in Hugh Walpole’s ‘I thought he should not offend the King if he could help it.’ In sense 11a, Pepys even used it without a negative at all: ‘One thing there is in his accounts that I fear may touch me; but I shall help it, I hope’ 

