It is so important to understand the context of lines such as these in order to work out what Shakespeare was meaning by having his character utter the lines that he wrote for them. Let us remember that in Act V scene 1, Touchstone is talking to his Audrey, and he mentions the fact that there is somebody else in the forest who loves her and who is competing for her affections and her hand. When William, the aforementioned youth, enters the scene, therefore, Touchstone utters the lines you have indicated above.

What he says to himself when he contemplates William, who quite clearly is meant to be shown as a rather dumb and not to bright individual is that he can't resist using his "good wits" to make fun of William and feels that, for a man like himself who is defined by his sense of humour and witty tongue, he just can't help but mess around with William because he is such an easy target. Touchstone is therefore shown to get his revenge on his rival for Audrey's affections.