Bugged: In the 18th century, the word 'bugaboo' became thieves' slang for a police officer. When people started putting up burgular alarms in the 1920s, burgulars becan calling these 'bugs' because they acted tike an automated policeman. It a home had a burgular alarm, it was 'bugged'.
Alcohol: The word 'alchohol' comes from Arabic 'al kuhul', a kind of make-up (which we still sometimes call kohl). As kohl is an extract, the word 'alcohol' started to be used to refer to the pure essence of something. In the 1600s, someone tried to find the pure essence of wine, which they called 'wine-alcohol', lated shortened to 'alcohol'.
Hocus Pocus: Thought to be a comedic mispronounciation of 'hoc est corpus meum' (this is my body) by people making fun of Catholic priests.
Guy: The word 'guy' (as in a man) comes from Guy Fawkes, who planned to execute the king of England. In celebration of the king escaping assasination, people burn effigies of him, usually made of rags. A man who looked scruffy/disheveled would sometimes be compared to these effigies, and be called a 'guy'. From there, the word eventually lost its negative connotation and now can refer to any man (and in the plural form - 'you guys' - to people of any gender).
-ly (the suffix on 'gently', 'slowly' etc.): This suffix comes from Proto-Germanic word *-līkaz, meaning body. It then started being used to describe things that had the body of (as in is similiar to) some other thing (something could be 'tree-like', for example), and later started to be applied to verbs (e.g. 'quick-like'). It was then shortened to 'ly'.
Robot: Robot comes from the slavic word 'robota', meaning forced labour or servitude. Its modern meaning comes from a play named 'Rossum’s Universal Robots' where manfactured beings are used as slaves to do stuff humans didn't want to do.
Malaria: 'Malaria' comes from Italian 'mal aria' (bad air) as it was once thought that malaria was caused by poisonous air coming from bogs and marshes.
Ampersand: An ampersand is the 'and' symbol (&), and was once considered part of the alphabet. People would say 'and per say and' (and by itself) to distinguish the character from the word. 'And per se and' later morphed into 'ampersand'.
Hotdog - In 19th century America, a widespread belief was that sausages were made from dog meat, so they started calling them 'hotdogs'.
Sinister - Sinister comes from a Latin word that means 'on the left side', as left-handed people were once associated with wickedness and other negative traits.
Vaccine - The word vaccine comes from the Latin word 'vaccus', meaning 'cow', as the first vaccine used a dose of cowpox to prevent smallpox.
Pants - There once was a legend about a man who was the emperor's doctor. After it was discovered that he was a Christian, the emperor sought to have him executed. The execution didn't succeed, and he forgave his executioner. This kindness earned him the name 'Pantaleon', which means 'all-compassionate'. Pantaleon became the patron saint of Venice, and 'Pantalon' became a popular Venetian name. Then, in the 16th century, Commedia Dell'Arte (comedy plays performed by travelling troupes; always involving the same stock characters) became popular. In these plays, Pantalone was the stereotypical Venetian character. He wore long trousers like the Venetians did, and these trousers became known as 'pantaloons', which was later shortened to 'pants'.
Toxin - The word 'toxin' comes from 'toxikos', the Greek word for things related to archery. This is because the ancient Greeks would poison the tips of their arrows when they went to war.