grep was from the ed command, g/re/p or Globally search for Regular Expression and Print.
yacc, of course, was Yet Another Compiler Compiler.
perl was Practical Extraction and Report Language (see also the comments).
roff was a simple contraction of 'runoff' (a similar system that existed on other non-Unix machines); troff was 'typsetting roff' or thereabouts; I guess that nroff was 'non-typesetting roff'; tbl is a simple contraction of 'table'; eqn is a simple contraction of 'equation'; grap is a simple contraction of 'graph'(and why it was felt necessary to drop the 'h' remains a mystery to me); pic is probably just a simple contraction of 'picture'; refer is a simple contraction of 'references'; and chem is a simple contraction of 'chemistry' (or 'chemical', or ...).
Ideally, the name was short, (almost) pronounceable, and (if you knew the secret) at least semi-mnemonic. That still applies today, to some extent. Names of programs are chosen on any basis the author cares to use.
Directly answering the question - what command is missing (and has a whimsical name)...
$ dribbler -h
Usage: dribbler [-hV][-s sleep][-f file][-m message][-o openstr]
-V Print version information and exit
-f file Write to named file
-h Print this help message and exit
-m message Write message on each line of output
-o openstr Flags passed to fopen()
-s sleep Sleep for given interval between writes
It is an unenthusiastic counterpart to yes. Whereas yes writes stuff as fast as it can to the output pipe, by contrast, dribbler sends a dribble of information, enough to keep a process on the receiving end a little bit active, but not enough to inundate the system.
Another command of which I'm fond is bombard:
bombard - bombard a program under test with signals
bombard [-s sig][-rvV][-t mintime][-T maxtime][-i iterations] -- cmd [args ...]
Bombard runs the test program designated by `cmd [args ...]' and subjects it to one or more signals. It can be used to do stochastic testing of the signal handling characteristics of a program.
The '-s' option specifies which signal is sent; the default is SIGINT. The '-V' option prints the program version number and exits. The '-r' option repeatedly sends the signal. The '-i' option specifies how many times the signal is to be repeated; the default is 1000 if '-r' is specified and '-i' is not. If the '-r' option is not specified, the flag has no effect. The '-v' option prints verbose information on the result of each run of the test program. The '-t' option specifies the minimum delay (in seconds) before signalling; the default is 0. The '-T' option specifies the maximum delay (in seconds) before signalling. The '--' separator is strongly recommended to separate the controls for bombard itself from the command and its arguments.
If only '-t' is specified, then bombard sleeps for the specified time and then sends signals to the process. If both '-t' and '-T' are specified, then bombard sleeps for a random amount of time in the specified range and then sends signals to the process.
Obviously, all the timing is subject to the idiosyncracies of the kernel scheduling algorithm.