After Muddy Waters topped the charts in 1954 with "(I'm Your) Hoochie Coochie Man", Bo Diddley borrowed the (now legendary) blues beat for his own "I'm A Man." This recording is basically Waters' response to Diddley. It is lyrically almost identical, except where Diddley sings "...made 21" Waters sings "...way past 21," and he adds the line "not B, O child, Y; that means Mannish Boy." While not as groundbreaking as Hoochie Coochie Man, this is one of the classics of blues. Jimi Hendrix covered the song in 1969 and The Grateful Dead did as well, in 1971.
Now when I was a young boy,
at the age of five
My mother said I was, gonna be
the greatest man alive
But now I'm a man,
way past 21
Want you to believe me baby,
I had lots of fun
I'm a man
I spell M,
AAA child,
NNN
That represents man
No B,
O child,
YYY
That means Mannish Boy
Waters uses the rest of the song to elaborate on the "I'm a Man" theme, describing his sexual prowess, his second cousin "Johnny Cocheroo", and the fact that he's a Rolling Stone, with lots of classic blues yelling in the background on almost every verse.