49. Implacable, Relentless

Implacable: Unable to be appeased
Relentless: Unyielding in severity

50. Ineffable, Indescribable

Ineffable: Too great to be expressed
Indescribable: Impossible to describe

51. Ingenuous, Naive

Ingenuous: Innocent and trusting
Naive: Lacking experience or judgment

52. Inveterate, Habitual

Inveterate: Long-established habit
Habitual: Done regularly

53. Laconic, Curt

Laconic: Using very few words
Curt: Rudely brief

54. Lethargic, Torpid

Lethargic: Sluggish and inactive
Torpid: Mentally or physically inactive

55. Malleable, Pliable

Malleable: Easily shaped
Pliable: Flexible and adaptable

56. Mendacious, Deceitful

Mendacious: Given to lying
Deceitful: Dishonest

57. Mitigate, Alleviate

Mitigate: To make less severe
Alleviate: To reduce pain

58. Morose, Sullen

Morose: Gloomy and ill-tempered
Sullen: Silent and resentful

59. Nefarious, Villainous

Nefarious: Wicked or criminal
Villainous: Extremely evil

60. Obsolete, Archaic

Obsolete: No longer in use
Archaic: Very old-fashioned