Learn English – Does (or did) “a trouser” or “a scissor” have a meaning

Etymonline has this to say:

-ics
in the names of sciences or disciplines (acoustics, aerobics, economics, etc.) it represents a 16c. revival of the classical custom of using the neuter plural of adjectives with -ikos (see -ic) to mean "matters relevant to" and also as the titles of treatises about them. Subject matters that acquired their names in English before c.1500, however, tend to remain in singular (e.g. arithmetic, logic).

So yes, at some point in history, there were such things as physic (meaning "natural science"), mathematic (meaning "mathematical science"), etc. that were later turned into plural forms but kept being treated as singular.

Edit: having looked in a few more places, it appears that in contemporary English, it still makes some sense to have both the suffix -ic and its plural form -ics. According to the Collins English Dictionary, the former has kind of specialized in forming adjectives, while the latter is happily forming nouns:

-ic
suffix forming adjectives

1. of, relating to, or resembling: allergic, Germanic, periodic. See also -ical.
   [...]

[from Latin -icus or Greek -ikos; -ic also occurs in nouns that represent a substantive use of adjectives (magic) and in nouns borrowed directly from Latin or Greek (critic, music)]

[...]

-ics
suffix forming nouns (functioning as singular)

1. indicating a science, art, or matters relating to a particular subject: aeronautics, politics
2. indicating certain activities or practices: acrobatics

[plural of -ic, representing Latin -ica, from Greek -ika, as in mathēmatika mathematics]

The key here is that they are not just two unrelated suffixes. Much rather, one is etymologically a plural form of the other. As the American Heritage Dictionary succinctly puts it, -ics is "-ic + -s".

Quick searching reveals these other names for Rohypnol (which is pronounced row-hip-nole, if you were wondering):

• rowies
• rophy
• ruffles
• roachies
• cock smack
• roofies
• ruffies
• ruff up
• rib
• roach 2 (R2)
• roche
• rope
• ropies
• circles
• circes
• forget it
• poppers
• forget-me-pill
• Mexican Valium
• mickeys
• forget-me-nows

It is kind of a long list but a few of these have more obvious origins. Roche is a common name for Hoffmann-La Roche who marketed the drug and easily accounts for the roach and roche variants. The drug itself has similar effects as valium, which would explain Mexican Valium and most of the other names are varied pronunciations of either roche or rope. The latter is "row-hip" without the "ih".

Bouncing from the row- and rope- pronunciations into a ruff- or roof- opening is understandable. If you transposed a few letters in the drug name you can find "roph" which is actually how I thought the drug was pronounced: "roe-fih-nal". I have no idea why I saw it that way, since it makes no sense at all. But there it is. Another potential mispronunciations is "roe-hif-nole" which again wouldn't actually work with the name but it sure sounds like a drug. Mushing "roe-hif" into "ruff" would be like mushing "row-hip" into "rope".

Another potential path is "ruff up" -> "ruffies" -> "roofies".

For what it is worth, some English dialects pronounce ruff and roof the same, which could account for the two variations: roofies and ruffies would sound the same as well.

So, all of that said, I wasn't able to find a history chronicling the street names of Rohypnol. Conjecture is probably the best we can do without finding a dedicated study but I don't think roofies is a drastic step from the drug name to bother looking into it further.