Weak One Way Function
Well, if "weak one way" means that you shouldn't be able to consistently find preimages, that is, inputs that generate a specific output, and $mult(a, b)$ is defined as the integer multiplication $a \times b$, then $mult$ would not meet that definition.

For an arbitrary output $C$, we can set $a = C$ and $b = 1$; hence $mult(a, b) = C$.