? is—that is, the first and last letters of quæstio (question). ! is . lo in Latin is the interjection of joy.
S is a Greek p (), the initial letter of paragraph. is used by the Greek grammarians to arrest attention to something striking (asterisk or star). is used by the Greek grammarians to indicate something objectionable (obelisk or dagger).
(See Marks In Grammar.)
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894