The punctus exclamativus (or punctus admirativus) first appeared in the latter half of the 14th century to mark the end of an exclamation. The Italian poet Iacopo Alpoleio da Urbisaglia claimed to have invented it. The influential Italian humanist Coluccio Salutati revived the exclamativus and its use spread in the 15th century.
In the American typographic and printing trades, the exclamation point was referred to as a “bang” or a “screamer.” One still occasionally hears these terms, as in “Postscript files always start with percent-bang-PS” (%!PS).
Traditional etymologies of the exclamation mark, recounted by the brilliant, amateur classicists, Alexander & Nicholas Humez in their book ABC et Cetera go like this:
“…the exclamation point … is derived either from an abbreviation of Latin interiectiō (interjection) or from the Latin interjection Iō! (‘Hey!’).” In their most recent book, On the Dot, the Brothers Humez explain that the exclamation mark was known in English as “note or mark of admiration (a straight-forward translation of Iacopo’s term punctus admirativus),” and the term “exclamation point” was adopted in the 17th century.
If you accept the traditional etymologies, the morphology of the exclamation point, as with the question mark, appears to boil down to the convenience of abbreviation. Medieval scribes stacked the i above the o, the o became a point, and thus evolved this indispensable, energetic punctuation mark.
Note: Avoid overuse.
Note: the term eleventy, or eleventies is an ironic twist on the habit of commenting with an over-use of exclamation points — typing ones and inadvertently lettting up on the shift key (e.g., !!!1!!!!111!!!). “Eleventy” or “Eleventy-one” refers to the ones that slip in between exclamation points.