Fugaz translates to "fleeting" or "ephemeral". It's a less common word than its synonyms, making it stand out in the lyrics.
In the song, the singer uses it to describe his lover's
Eduardo Costa sings about a love that keeps slipping through his fingers. He's already fallen — "É certo que eu me apaixonei" ("It's certain that I fell in love") — but she stays hard to hold onto: "Você é tão fugaz no sentimento" ("You're so fleeting in feeling"). Her indifference wounds him, and he admits they're standing "quase a beira do fim" ("almost on the verge of the end").
Rather than walk away, he asks her to notice him — "Por que você não olha pra mim" ("Why don't you look at me") — and just wants things set right: "Põe de vez essa cabeça no lugar" ("Finally get your head straight"). The chorus lays his plea bare: his heart is asking for attention, he misses her, and if they can sort themselves out, this love can still work — "Como eu te amo, não vivo sem você" ("How I love you, I don't live without you").