Low-level situational speech taboos are still relatively active in Japan. For example, you aren't supposed to say "cut" (kiru) at a wedding — even ceremonial cake-cutting is referred to as "inserting the blade" (入刀) — and it's bad manners to say "fall" (ochiru) or "slip" (suberu) (i.e. "fail") to someone about to take an important exam. The number of people who don't really care about this stuff is growing, but they're still common wisdom, useful for driving comic book plots and the like.

According to Magic and astrology in the Sengoku period (呪術と占星の戦国史) by Owada Toshio 小和田哲男, similar taboos were in place back then too. Obviously, words like "death," "lose," and "defeat" were right out, as were homonyms like "four" (shi, same as "death"; actually, shi is still avoided today — my building has an apartment 103 and 105, but no 104). If you absolutely had to write the number 4, you might do it by writing two 2s together, either side-by-side (like ﾆﾆ) or squished closer and angled together, like × with the center missing.

Another forbidden word was hiku, as in "pull back," "retreat." Even gestures implying this word were frowned upon. For example, here's a passage Owada quotes from an Imagawa clan etiquette manual (the 今川大双紙, compiled by Imagawa Ryōshun 今川了俊):

出陣の時酌報事、御酒参らするには、左の膝をつきて参らする也。膝をばなをすとも、足をば後へひかぬ事也。

When departing for battle, the ladler shall kneel on his left knee to serve the sake. Though he may adjust his knees, he shall not pull back his feet.

Normally, after serving the sake, you would knee-crawl backwards out of respect. This was not permitted. You had to knee-turn and face forward as you left.

"North" was yet another pre-battle taboo, because the kanji, 北, also means "flee" or "turn one's back." (The character was originally drawn as two people with their backs turned to each other, it seems. Note also that this is why the character for "back" [the body part] is 背: it's 北 + the "meat moon" body-part radical.) Not to mention the fact that corpses were placed with their heads facing north, and this was known as kitamakura (north-pillow), an undesirable way for the living to sleep. All in all, a bad bundle of imagery for superstitious soldiers, and so you were not to leave your armor facing north, face north while putting it on, or dismount a horse from the northern side. It is for this reason that the armies of Japan were never able to mount a successful campaign against Santa Claus.

However, north was good for one thing: disposing of enemy heads, once they had been displayed to the satisfaction of your superiors. You wanted them to flee, after all, not stick around haunting you.

(The post-battle display of trophy heads, by the way, was known as the kubi jikken 首実検. Meanwhile, in modern Japanese, jikken means "[scientific] experiment." The shared etymology is in the characters: "truth[fulness] inspection," and it's obvious when you think about it — did you really kill that general? do the rats really grow extra tails? — but it still gives me a little "Re-animator" thrill sometimes when I see it in print.)