役不足 (yakubusoku) is one of the most misused Japanese words by native speakers. If you notice that your Japanese friends use this word in a wrong way, you may want to enlighten them with providing the correct definition described below.
If a position or job is 役不足 for a person, that means he/she is overqualified for it. For instance, a supporting character is definitely 役不足 for Hollywood stars. It can be also used when over-engineered or extravagant things are used as a sledgehammer to crack a nut, as in “このハイテクな掃除機を狭い部屋に使うのは役不足だ” meaning “It is 役不足(overkill) for this high-tech vacuum cleaner if you use it in a small room”.
Misuse of 役不足
役不足 is one of the most misused Japanese words by native speakers. According to a survey conducted by 文化庁 (Agency for Cultural Affairs) , only 40.3 % of Japanese people recognize its actual meaning, whereas 50.3 % of people misinterpret 役不足 as the opposite meaning, that is “someone is underqualified”. In particular, they use it to sound modest when they feel they are underqualified for a job or position. In this case, they confuse it with “力不足“, that means “not have enough ability to do something”.
Remember that 役不足 is used to describe something that is too trivial for a high-level person or thing, while 力不足 is used to describe a person who lacks enough ability to do something. Therefore, it is fine to use 力不足 to describe yourself, but not 役不足.
役不足 consists of the two words “役” meaning “role” or “position” and “不足” meaning “shortage” or “not enough”. Therefore, 役不足 describes a role that is not important enough for a high-level person or thing. However, many people misinterpret that it would describe a person who does not have enough experience or ability to fulfill a role.
Such mundane work would be 役不足 (too trivial) for him.
MacBook ProをYouTube やTwitterだけの為に使うのは役不足だ
It is 役不足 (overkill) if you use MacBook Pro just to browse YouTube or Twitter.
Ex3 [Wrong Usage]
I am 役不足 (underqualified, not experienced enough) for such an important task.
(Note) To correct this sentence while keeping its meaning, replace 役不足 with 力不足.
Synonyms: もったいない (mottainai; one’s ability is not utilized properly)
Synonyms in English: overkill, too trivial, a thing for which one is overqualified,
Related words and phrases: 力不足 (not have enough ability)
Reference (in Japanese)
 文化庁; http://www.bunka.go.jp/pr/publish/bunkachou_geppou/2012_02/series_07/series_07.html