The “Head Pad” (PAD(パット)長, Paddo-chō), or “Pad Chief”, was the derogatory nickname for the Touhou Project character, Sakuya Izayoi, as she is the 'head maid' (メイド長, Meido-chō)¹⁾ of Koumakan. It was a cheap joke that claims that she wore breast pads in Touhou Suimusou.

In the early 2000s, people were bound to compare how characters looked in the mainline games to how they looked in Touhou Suimusou, so people would joke that Sakuya Izayoi wore breast pads as an explanation. Obviously, we know that “Alphes” is not “ZUN” and that artistic liberties can exist, but this was during a time when cheap jokes were commonplace on bulletin boards, so people simply ran with the idea.

The joke's simplicity caused fan artists to use it as a cheap gag or signal 'breast envy' which could further the idea of 'femininity', then it became an overdone joke as people spammed it everywhere without reading the room (e.g. unprompted 'pad' comments). Eventually, the joke went away in the late 2000s as people were now propping up the 'rivalry' with Sanae when Touhou Seirensen released in 2009.

First off, we have the obvious antithesis of “Head of the Big Boobs” (巨乳長, Kyonyū-chō), or “Busty Chief”, to argue that her breasts are real, plus her more recent mainline appearances eventually affirmed it was a skill issue. Naturally, this sort of thing can get vaguely 'erotic' or 'sexual' when it comes to 'disputing' the claim, plus some people generally love boobs and the maid outfit uniform fetish exists for a reason.

After this, you have the synthesis of “Head Sarashi” (サラシ長, Sarashi-chō) which proposes that she binds her massive chest with sarashi in confrontational scenarios (i.e. the mainline games), but doesn't bind for casual scenarios as Touhou Suimusou was supposed to be casual banquet. Of course, an explanation like this isn't necessary and sorta falls apart anyways.

• Considering that Sakuya is often called “perfect and elegant” (完全で瀟洒, kanzen de shousha), several backronyms playing around with this descriptor would emerge. This includes: “perfect and delicate”, “perfect and dashing”, “perfect and dainty”, “perfect and delectable”, and so forth.
• On August 11, 2007, the Nae-Radi DNA Special before Comiket 72 with ZUN, the subject of Sakuya's breast size had actually came up, but ZUN ended up dodging the question.²⁾
• Long after it had been phased out, the English-speaking community has invented a semi-serious headcanon that Sakuya is a transgender woman, having to rely on padding her chest until Eientei set up shop in Gensokyo, following the events of Touhou Eiyashou, and provided the medicine.

• Meiling's Name

• Pad Chief on Pixiv Encyclopedia
• Pad Chief on Niconico Encyclopedia