Translates into "Cactus on the forehead". The Nopal is a cactus that is native and synonymous with Mexico.
Depending on the context, it's a term used for Mexican-Americans who have homogenized, or "sold out" themselves into American culture.
1.) Tongue-in-cheek term used by Mexicans to Mexican-Americans. Used by mainly by Mexican immigrants to tease Mexican-Americans who have become too "Americanized" and disconnected from their ancestral culture, either because they forgot how to speak Spanish fluently, or no longer observe Mexican traditions, customs and celebrations.
2.) A slur used by Mexicans to Mexican-Americans who are "American First". To some Mexicans, Mexican-Americans who identify themselves as patriotically American are "selling out" to "white oppressors" since patriotism is not a value you can have if you're not white. They also accuse Mexican-Americans that do so as deriding their Mexican heritage or roots.
The type of American-born 'Mexicans' that use such a term are the ones that hold 'anti-imperial' sentiments against "Gringos". They feel that "Gringos" have no rightful claim to enforce American borders since they "stole" the lands. This, even though the only reason they live in America is because American colonization made the "stolen lands" a much better place in comparison to the rest of Mexico, and offered more opportunities for them.
Definition 1: Hey mijo, why are you speaking to me in English? Are you forgetting how to speak Spanish? We taught you how to speak fluent Spanish in the home. Are you forgetting where you came from, or are you forgetting on purpose? If so, look in the mirror and you see the 'Nopal en la frente' to remind you of your roots.
Definition 2: Did that American-born 'Mexican', who was at the protest wearing an Aztec feathered headdress he bought on Amazon (while also flying a Mexican flag), accuse me of "selling out" to my "Gringo oppressors" because I told them we should have borders? And that I should see the 'Nopal en la frente' because I'm not white? What a delusional and dumb hypocrite. He knows the only reason his parents moved here is because the US was a better country to live in than Mexico, and that if the "stolen lands" still belonged to Mexico, it would be just as underdeveloped as the majority of Mexico. What a militant idiot.