Archaic English, meaning "called" or "named".
"It is full fair to ben yclept madame" - Chaucer
"To be called 'Madam' is acceptable". Perhaps in Chaucer's day, it was. These days, "Madam" has many uses, and has fallen into disfavour. The word "yclept" (or "ycleped") has fallen so far from favour as to be used only in jocular or poetic senses. I call this a shame.
It is pronounced with a long "e", as in he. The etymology is seemingly straightforward enough, taken from the Middle English verb clepe (meaning to call), and originating with the Old English (possibly Indo-European) root of cleopian . Dear Webster mentions that the y- prefix was used to create the past participle of a verb (the equivalent of the German prefix ge- or the modern English suffix -ed). So there we have it.
The word is little-used nowadays, although Michael Quinion mentions that it still pops up from time to time in journalistic pieces, or where someone seeks to make a moderately humorous point about a name or nickname. Even though it ceased to be in common usage in England in the 13th century (dying out from North to South), it still appears in most modern dictionaries.
More modern uses include snippets such as "...the unfortunately yclept basketball player just makes matters worse: ‘David Putz dribbles away...’” (from the Jerusalem Post in 1997) . The 15th-century poet and mystic Gavin Douglas also used it in his work, but to little avail. It remains a curiosity now, largely neglected and possibly best avoided.
bookw56 says: Actually, I have to say that I still use "Ma'am" to talk to older women, despite being from the Northeast
My housemate Tim says "I have a computer named yclept"