Why do we use “conmigo” but not “sinmigo”?

Question

In Spanish we typically use "conmigo" for "with me" but "sin mí" for "without me".

What are the origins of the word "conmigo"? How did it come to be preferred over "con mí"? Why do we continue to prefer "sin mí" over something like "sinmigo"?

Answer 1

Origin of conmigo

Conmigo comes from prefixing the Latin mecum ("mi con") with con- after it had evolved morphologically to the point where the preposition needed to be reinforced (a positive analogue of the Jespersen's Cycle):

me+cum → mecum → micu(m) → migo → con+migo → conmigo

Historical absence of sinmigo

The expected Latin grammar of "with me" is cum mē, and as such the common form mēcum has multiple proposed explanations. None of the hypothesised motivations apply to sine me ("sin mi") and hence there was no reason for a parallel sine form to develop in Latin.

Modern prevalence of sinmigo

However, given that conmigo is lexicalised in Spanish, it has been hypothesized that analogous forms *sinmigo, *sintigo etc may develop:

In conclusion, we might speculate on the future of the lexemes conmigo, contigo, consigo. Will they eventually follow connusco, convusco and regularize to con mí, con ti, con sí, or will the pressures of the language academies preserve them with the apparently meaningless morpheme -go? Or will this morpheme spread through lexical polarization, i.e., conmigo ~ *sin migo, contigo ~ *sin tigo, and then on to other syntagmas, *de migo, *de tigo, *a migo, *a tigo, etc.?¹⁰ Unfortunately for diachronists of the present, only time will tell.

• Excessive Analogical Change as an Impetus for Lexical Loss: Old Spanish connusco, convusco (1990)

And indeed this has been noted in some Argentinian and Mexican speakers (though this is heavily stigmatised):

Pues en México, se oye "sinmigo" y "sintigo" pero también se oye "haiga."

Es usado, pero no correcto.

This is not to be confused with its use ironically (e.g. "Sinmigo", Mr.Kilombo ft. Rozalén).

Answer 2

The preposition "con" (with) comes from the Latin preposition "cum" and in that language when using pronouns, they would put "cum" at the end so instead of saying "cum me" they would say "mecum" (with me), tecum (with you), etc. Over time, people who spoke vulgar Latin started saying "cum mecum" to say "with me" which evolved to become "conmigo" in Spanish. We also have "contigo" (with you) and "consigo",although this last one is only used with the word "mismo" (self) in sentences like "él se enojó consigo mismo" (he got mad at himself), "ella estaba hablando consigo misma" (she was talking to herself).

On the other hand, "sin" comes from "sine" (without) and it was always put before the pronoun in Latin, "sine me" (without me), "sine te" (without you) and it became "sin mí", "sin ti". Although I've heard young teenagers saying "sinmigo" and "sintigo" a couple of times before but in a playful manner, completely aware that it was wrong.