Cishet, or cishetallo, is a term used by LGBTQ+ people to easily describe people who are not members of the community in any way, shape or form. This means people who are simultaneously cisgender, heterosexual, heteroromantic, allosexual, and alloromantic. The term originated within the trans community, particularly among straight trans people. It was used as a way to specify something who is not part of the LGBT+ community by virtue of sexuality or gender.
Exclusionists
Exclusionists have often used the term cishet to try to erase a-spec people (particularly heteroromantic asexuals and aromantic heterosexuals) from the community as they use the term to imply that a-specs aren't inherently LGBT+ by being a-spec. The term 'cishetallo' preferable to make it explicate that one is not referring to a-specs.