From the Super Mario Wiki
Squeaky Mallet is Luigi's Offensive Power Shot in Mario Power Tennis and Mario Tennis: Power Tour. Luigi pulls out a huge Hammer similar to Mario's Iron Hammer and hit the ball with great force, leaving a path of small green, yellow and pink stars. This will knock the opponent back about a quarter of the court and cause the opponent to lose whatever Power Shot they may have stored. If Luigi returns another power shot with the Squeaky Mallet, he will not suffer from any knockback.
Although not as fast and powerful as Mario's Offensive Power Shot, it has better recovery time and is a bit more curved. The stun time after contact is also greater. However, on rare occasions, the ball could go out of bounds by this power shot, which would hinder Luigi more than help him.
The Hammer that Luigi wields may be a Mini-Mega Hammer from Mario Party 4 with a star painted in the center instead of a Mushroom. The green and pink stars may also represent the Super Mega Mushrooms and Super Mini Mushrooms.