[fouc]

  test/test_corpus.cc
  364:        auto checkPragma = [&](string ext) {
  368:                    << "Missing `# typed:` pragma. Sources with ." << ext << ".exp files must specify # typed:";
  377:            checkPragma("cfg");

A quick look at the source shows that it may have been called pragma at one point.

EDIT: I'm guessing "sigil" was chosen because it's closely matching the "signatures" or Sigil.sig method name?

[ptarjan]

Nice spelunking! Yeah, this word means nothing really. Sigil, pragma, directive, typed comment, whatever. Feel free to call it "dohicky" if you'd like.