"Showing off is also a kind of Shirk (idolatry or polytheism)because when one tries to show-off, he tries to please someone other than Allah. We should also remember that Shirk is one of the major crimes that Allah does not forgive, unless we give it up and seek Allah`s forgiveness."
source: [Kazi, Mazhar U., Guidance from the Messenger (1990)., India Press Message Pub., ICNA, pp.76]
In Islam, shirk (Arabic: شرك širk) is the sin of idolatry or polytheism. i.e. the worship of anyone or anything other than the singular God, or more literally the establishment of "partners" placed beside God.
Within Islam, shirk is an unforgivable sin; God may forgive any sin except for committing shirk.
Verily whoever sets partners with Allah, then Allah has forbidden paradise for him and the Fire will be his abode,
[Noble Qur`an, Sura Al-Ma'ida - verse 72]
The Prophet ﷺ (peace be upon him) was asked: 'What is the greater sin?' he said: "To ascribe partners to Allah even though he created you," [Transmitted by al-Bukhari]