Easy; simple. An adjectival expression meaning something is very easy or no trouble, commonly used in spoken Japanese.
この問題は小学生にとってわけないよ。
This problem is nothing for an elementary school student.
彼に頼めば、そんなことはわけないだろう。
If you ask him, that should be no problem.
準備をしっかりすれば成功はわけない。
If you prepare well, success is easy to achieve.
A contraction of わけがない meaning 'there's no way that...' or 'it is impossible that...' used to express strong denial or disbelief.
彼がそんな嘘をつくわけないと私は信じている。
I believe there's no way he would tell such a lie.
急にお金を全部返せるわけないだろう。
There's no way he can suddenly pay all the money back.
そんな簡単に成功できるわけないよ。
There's no way you can succeed that easily.