Some people said that wearing of shirt is Kufr. What is the origin of the shirt? Is it true that it originated from the Kafirs?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Shirts in their known shapes and forms these days and which are worn with trousers are lawful if they conform to Sharia rules. So, they should cover the Awrah and should not be made of something forbidden like silk or gold. Wearing such shirts is not forbidden, let alone an act of disbelief. Imitating disbelievers which is forbidden is imitating them in their symbols, like wearing priests' clothes.
al-Hafiz Ibn Hajar says explaning the Hadith that proves prohibition of using saddlecloth: 'That was their rite when they were disbelievers. Now that this is no longer one of their rites, then it is no longer dislikeable to use it' . This is so, if it originates from disbelievers. But, generally the shirt was known during the era of the Prophet (Sallallahu Alaihi wa Sallam). Abu Dawood reported from Umm Salamah (Radiya Allahu Anha) that she said: "The shirt was the dearest garment to the Prophet (Sallallahu Alaihi wa Sallam) " . But this kind of shirt is long enough to reach close to the ankles.
Allah knows best.