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If the holder's current species is Cubone or Marowak, its Attack is doubled.
Thick Club is an excellent item for Marowak to use as it doubles Marowak's already decent base 80 Attack stat. With STAB Earthquake, Rock Head Double-Edge, Swords Dance, and over 500 Attack, Marowak is a huge offensive threat. Attack boosts from Swords Dance are independent of Thick Club's boost, so a +2 Marowak will have four times the Attack of one without any boosts.
Marowak does not need to resort to Choice Band to hit hard, as holding Thick Club allows it to switch its moves as needed and provides an even greater power boost. Needing less Swords Dances to hit spectacular Attack scores is also an advantage, as Marowak will have few chances to set up. It is also not left entirely helpless if it loses its item to Trick or Knock Off, although it will be much less powerful, with little redeeming Speed or bulk.
However, Marowak is quite slow and has many weaknesses; considering these, its defenses, especially when without EV investment, are nothing to be proud of. This can be remedied by Trick Room, but Trick Room only lasts five turns, and anything which can survive its onslaught can at least cripple or beat it. Without Leftovers, Substitute will wear it down fast, and it has no way of recovering damage taken. It is also vulnerable to status, especially Will-O-Wisp, without the use of Lum Berry.
Marowak, and therefore Thick Club, is only really viable under Trick Room, or when passed Speed. Paralysis support is also useful to clear the path for Marowak to actually get some hits in before it is KOed.
In the Little Cup metagame Cubone is a great user of Thick Club, like Marowak in level 100 play. Cubone reaches 30 Attack, higher than any other LC legal Pokemon, has Rock Head Double-Edge, and a very strong Earthquake. However, it experiences the exact same problems as its brother, Marowak. With its low Speed, vulnerability to priority moves, and weakness to the common Water-type attacks in LC, Cubone has a hard time lasting long enough to deal a significant amount of damage to the opponent’s team. To abuse Cubone to full effect, paralysis, Agility Pass, or Trick Room support is generally recommended.