Origin of terms "flag", "flag manifold", "flag variety"?

Armand Borel's Bourbaki Seminar 121 Groupes algébriques is from 1955, and uses "drapeau" (page 7). (It's online at archive.numdam.org.) This may not be the earliest occurrence, but there is a good reason for attention to the full flag variety in this context (the theory of Borel subgroups).

The concept traces back some way, to Ehresmann's thesis in the 1930s; Kolchin's work (the Lie-Kolchin theorem) uses the non-intrinsic language of upper triangular form. Hodge & Pedoe talks about Schubert spaces in general, which would be natural in the enumerative geometry tradition, for which full flags is just one of the cases.

Edit: A further data point is Chern's paper On the Characteristic Classes of Complex Sphere Bundles and Algebraic Varieties (1953), which relies on Ehresmann's work to some extent. The word "flag" is absent (though used by Chern discussing it in his Selected Papers).

I think the concept may date back to René De Saussure (1868-1943). He was interested in the Euclidean geometry of 3-dimensional space and used the term "géometrie des feuillets". I think this may have been his doctoral work. The work was criticized by Eduard Study because de Saussure failed to reference Study! Despite teaching Math at the University of Geneva de Saussure seems to be more famous for his work on Esperanto.

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From his papers:

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@Jim, I always asked myself the same question. You say that the "notion of flag variety or flag manifold evolved from the older and still somewhat mysterious use of the term flag in projective geometry, for instance to refer to an incident point-line pair." Once I saw a drawing in an expository article that solved that mistery for me: picture a point in the projective plane as the corresponding line in space, and a projective line as the corresponding plane in space, then an incident point-line becomes a line waving a plane in space, like a flag :)