sick, sic

Sick or sick means ill or not well, physically, or unbalanced or disturbed, mentally--
He called in sick because he has the flu.
He is still feeling too sick to go to school.
This is the product of a sick mind.

Sic or sic ( rhymes with sick ) means to set a dog or guard animal upon, by ordering it to attack--
He got rid of the obnoxious salesman by threatening to sic the dog on him.
That dog has been trained to attack, and the owner may sic him on you if you bother him.

Now that you know that, you can say--
"He claims he is too sick to go out, but he is really afraid that our neighbor will sic that dog on him."
Note--Sic is also Latin for "thus", meaning "like this", and is used when quoting a passage from another work, to show that any unusual spelling or grammar is as found in the original, and not an added error made by the person doing the quoting--
"Ain't [sic] she sweet..."
"Sometimes I sits [sic] and thinks..."