Theory:

Macavity, Macavity, there’s no one like Macavity,
He’s broken every human law, he breaks the law of gravity.
His powers of levitation would make a fakir stare,
And when you reach the scene of crime — Macavity’s not there!
You may seek him in the basement, you may look up in the air —
But I tell you once and once again, Macavity’s not there!
Explanation:
 
The second stanza begins with a refrain: "Macavity, Macavity, there’s no one like Macavity". Despite Macavity being a master criminal, the refrain suggests that the speaker is an admirer of his skills. The speaker claims in pride that Macavity is unique and that there is no one like him.

Macavity is known for defying laws, and he has disobeyed every human law, including the law of gravity.

The line is a reference to the cat righting reflex. It talks about the capacity of a cat to position itself as it falls so that it can land on its feet.
 
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A cat lands on its feet during a fall
 
The speaker continues to explain how even a fakir would be taken aback by his ability to tackle a fall. Fakir, who is famed to be a wonder-maker, can never recreate the cat's righting reflex.
 
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A fakir
 
Here, the speaker has exaggerated the cat’s ability by calling it “powers of levitation”.  The speaker doesn’t mean that the cat can stay afloat in the air, but instead, he intends to emphasise the cat’s ability to fall on its feet.
 
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Levitation is an act of something floating in the air
 
Moreover, Macavity can display more tricks: Macavity can easily disappear into thin air without leaving any trace behind. He may commit a crime, but when you get to the crime scene, Macavity is nowhere to be seen!
 
You can look for him in the basement or up in the sky, but Macavity is so skilled that he would have vanished from the site.
 
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Macavity is quick to vanish from the scene of crime
 
Meanings of difficult words:
 
S.No
Words
Meanings
1
GravityThe force which causes things to drop to the ground
2
LevitationThe action of rising or causing something to rise and float in the air
3
FakirA Muslim or Hindu wanderer, often one reputed to perform magic, miracles, and wonders
4
BasementThe floor of a building which is partly or entirely below ground level
Reference:
National Council of Educational Research and Training (2008). Honeydew. Macavity: The Mystery Cat - T. S. Eliot (pp. 50-51). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.