OK, OK, I surrender. "Neb" standing on its own as an answer to a question is an abbreviation of a negative sentence, complete with verb and negator; "nid oes neb", "nid yw neb", "ni bu neb", "ni byddai neb", "ni laddwyd neb", "ni ddiddorwyd neb"... whatever the question called for. The same goes for "dim", except that a native speaker will often keep the mutation of the initial d- which would have been demanded by the context. Oes dadl o gwbl? Dim. (= Nacoes, nid oes affliw o ddim.) "Dim neb" is heard; well, I have heard it; I don't know that I ever expect to see it written in any formal context. It think that it is a dialect form, and I do not think that it is what BBC English By Radio call "acceptable". It seems to me to be an attempt to give a negative context to the "neb", compounded by the misunderstanding about the positive nature of "dim". "Mo" and "Smo" were contractions, of "ddim o" (mo'r fath beth = ddim o'r fath beth) and "[ni]does dim o" and they have a rhediad which parallels that of "o" (mohonof, mohonot, mohono, mohoni and so on). It seems to me that they would be natural in a context where the "o" would be present, such as quantity of substance or genetives that indicate parts, but they are used more generally (smo fi isio mynd = nid oes arnaf eisiau mynd; smo fi'n gwybod = nid ydwyf yn gwybod) and I do not know whether this counts as a dialect use, but I don't think it is "acceptable" formal use. Smo fi'n gwybod, 'achan. S'dim o fi'n gwybod, weithiau chwaith. "Pam nad yfi di mo'r coffi?" (am ei fod o'n oer) "'Sgen i mo'r llythyrau." "Ddarllenais i mo'r rhan olaf o'r neges" seem fine as "...(d)dim o'r..." but whilst I typed "[nid oes] gen i mo eiriadur..." last night because that is what I would have said, I thought better of leaving it in writing and changed it to "...gen i ddim geiriadur...", the posh affected so and so that I am. g