Reading 九十九 as "tsukumo"

Question

Why can 九十九 be read as "tsukumo"? I don't understand how those kanji could be read that way.

Answer 1

I looked up in my etymology dictionary (小学館's 日本語源大辞典) :)

The answer goes like this:

1. つくも was originally a name of a kind of plant (modern standard name: フトイ; English name is softstem bulrush or great bulrush according to Wikipedia).
2. A compound word つくもがみ < つくも + かみ "disheveled white hair (especially of old women)" was coined, because of its resemblance to the plant. This word is seen in 伊勢物語 from the 10th century.
3. The kanji 九十九髪 came with the word. This kanji was actually kind of a word-played ateji intending to mean "white hair". Its logic was like a riddle and went like this: white = 白 = ( 百 without 一 ) = 100 - 1 = 99 = 九十九.
4. Afterwards [九十九]{つくも} became to be used without 髪 in some other words (mainly proper nouns), like 九十九湾 "Tsukumo Bay" in Ishikawa pref., or several rare family names like 九十九, [九十九里]{つくもり}.

So, how amusing. In modern Japanese, [白寿]{はくじゅ} means "the celebration of 99th birthday", and it must be under the same logic (in the reverse way). But few people know why 九十九 reads つくも, I guess.

Answer 2

Well, the pronunciation of kanji can be really irregular. In many cases the only thing we can do is to remember them one by one.

I think the second explaination in @isayamag post is right. In http://ja.wikipedia.org/wiki/%E4%BB%98%E5%96%AA%E7%A5%9E,

「付喪」自体は当て字で、正しくは「九十九」と書き、この九十九は「長い時間（九十九年）や経験」「多種多様な万物（九十九種類）」などを象徴…

It says that 九十九 means so many years and much experience as well as various categories of things. According to many Japanese stories/animes, tsukumogami origin from either daily necessities or instruments or something else, and they have been existed for long time.

九十九->つくも is just like 八百万->やおよろず. In short, they both means the number(of spirits) is great.