I assume that you are asking about the story "The Monkey's Paw." So I will edit the question to show this assumption. I hope I am right.
In this story, Sergeant-Major Morris has warned the White family that they need to be careful about their wishes. The first thing that Mrs. White says is that she might like to have four pairs of hands.
When she says this, the Sergeant-Major tells them to wish for something sensible. He is saying two things in the lines you mention:
- They shouldn't really wish -- it will come out badly. That's why he says "if you must wish."
- I think he is saying that strange wishes like that are going to get fulfilled but that they will be fulfilled in terrible ways. That's why he's so alarmed when she makes that wish.
As it turns out, even a sensible wish for money turns out badly.
One of the most relevant lessons of life for our times and young people, especially, is used in "The Monkey's Paw" by W.W. Jacobs. This lesson is expressed in the old maxim, "Be careful what you wish for; you may get just receive your wish." In other words, importantly, a person should always think through his/her desires and choices. Otherwise, the consequences may not be what one anticipates.
The Whites' wish is a very pregnant example of the value of the old maxim. Not once, but twice, do the Whites not consider the full ramifications of their wishes despite the old soldier's warnings and his regarding of Herbert White
in the way that middle age is wont to regard presumptuous youth.
Sergeant Major Morris understands the potential for Herbert White's not making a "sensible" wish. His statement is, of course, a foreshadowing of the tragic end of Jacobs's story.
Sergeant Morris is aware of the evil implications of using the monkey's paw. He has witnessed the disastrous results that have occurred when a person he knew had made wishes on it before. He warns the Whites not to use the paw, but he is well aware of the temptation that the paw has over people. He has warned them, but since he knows they will probably make wises anyway, he is trying to guide them to be careful of what they wish.
In the case of the monkey's paw it appears that any wish would not be a good one to make. All the wishes seem to turn into something evil. The Whites do not wish extravagantly. They are careful with their first wish, but it still ends up badly for them.