guard

1 guard /ˈgɑɚd/ noun
plural guards
1 guard
/ˈgɑɚd/
noun
plural guards
Britannica Dictionary definition of GUARD
1 
[noncount] : a state in which someone is carefully looking for possible danger, threats, problems, etc.
2 
a  [count] : a person whose job or duty is to watch and protect someone or something
— see also security guard
b  [count] : a group of people (such as soldiers) who protect a person or place
c  the Guards : soldiers who protect a king or queen — see also color guard, honor guard, old guard, rearguard
3 
[count]
a  : something that keeps an unwanted result or effect from happening — often + against
— see also safeguard
b  : a special part or device that protects someone or something from injury or damage
4 
[count]
a  American football : either one of two players who play in positions on either side of the center
b  basketball : either one of two players who usually play away from the basket and control their team's play when they are trying to score points — see also point guard
5 
[count] British : conductor 2

off guard

: in an unprepared state : not ready

under guard

: in the position of someone (such as a prisoner) who is being watched by a guard
2 guard /ˈgɑɚd/ verb
guards; guarded; guarding
2 guard
/ˈgɑɚd/
verb
guards; guarded; guarding
Britannica Dictionary definition of GUARD
[+ object]
1 
: to watch (someone) in order to prevent escape
2 
: to protect (someone or something) from danger or attack
3 
: to be careful about not telling or talking about (something, such as a secret)
4 
sports : to try to keep (an opponent) from scoring (especially in basketball)

guard against

[phrasal verb]
guard against (something)
: to try to keep (something) from happening